I’ll start.
干天 (dry weather)
Most Japanese people have never heard of it.
I’d like to make a list so I can avoid embarassing myself in a real conversation.
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If you like you could also install this script:
During the reviews it will show you for each vocab item whether it’s common or not (according to jisho.org).
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里心, apparently. 
On a more serious note, astonishingly, 鰐蟹 is practically un-heard of.
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That’s weird, my Japanese teacher just used it in our lesson this past Thursday!
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My colleagues’ Japanese teacher who let me sit in a few sessions didn’t recognize 欠かす. I figured it was my pronunciation and explained after class what I was going for (even wrote it out on paper) and she gave it the side-eye and said people don’t really say that.
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I don’t know about saying it verbally in everyday conversation, but that word is far from being rare in the wild, let alone “extremely rare”. I’ve seen it 6 times in my current book alone lol
家で予習復習を欠かさないお陰もあり、基本的には学生の本分で困る事はない。
この余裕の発言は真昼が日頃から努力を欠かしていないが故のものだと分かっているので、さして何も思わない。
努力を欠かさないから常に首位をキープしているのだろうが、その努力を欠かさないところが真昼のすごさでありえらいところだろう。
一年時で既に陸上部のエースと呼ばれていた優太は監督やコーチからも期待されているようで、その期待に応えるように努力を欠かしていない
もちろん生まれ持ったものもありますが、努力を欠かした事はありませんし、時間と労力をかけて築き上げたものです
Its even more common if you also count 欠かせない which is also not particularly rare and just a conjugation.
But overall, I mean from a practical standpoint most rare words are just the words that are only used in contexts you don’t put yourself around. Since it seems like you specifically want words that have a definiton thats used but aren’t used because another word/phrase with that definition is more commonly used instead…taking a quick look I found a few
悪癖 is much less common in speech than 悪い癖
総体的 is much less common than 全体的
任意 is much less common than just saying what it means with a prase or something
If you just want to say cloth, don’t use 織物 without knowing what it means. WK definition kinda misleading for that one
言葉つき isn’t used as much as alternatives
I can’t sit here and go through all the words, but after skimming those are a couple
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Not absolutely sure about this one, but I think 博打 refers to old-style gambling like mahjong. In a course I heard ギャンブル being used for modern casino-style gambling.
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This has happened to me twice, when I was talking to some Japanese people they didn’t know what 桜肉 and 聴力 smh, Japanese people not knowing Japanese, smh
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No wonder I felt this word was strange and turned into a leech for me, if it’s actually quite uncommon. 
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I’ve heard the same thing (directly from a native speaker). 博打 isn’t really used anymore.
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Wouldn’t that refer to traditional gambling yakuza people might be doing, though? I did some googling and that’s what I found online when looking for 日本の博打 (博打 alone gives tons of results in Chinese about some gaming celebrity 
)
Definitely not something one would bring up in a Japanese class, otherwise one might get looks of concern from the teacher 
.
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Since its hard to know what words are super rare if you don’t have a decent sample size of reading, I would suggest frequency dictionaries. They give you an idea for how much a certain word appears within the given medium. If a word appears with very low frequency and has a high frequency word as a definition, only use the high frequency one.
https://drive.google.com/drive/u/0/folders/1tTdLppnqMfVC5otPlX_cs4ixlIgjv_lH
I see people often say “Japanese people I asked didn’t know xyz” and sometimes its totally legit but sometimes it can be misleading. Many words mentioned I see mentioned are words I have come across several times and are a necessity to learn for advanced reading imo, but not so much for usage in conversation. If you want to just focus on speaking, I would use the netflix frequency dictionary for obvious reasons. Usually its pretty easy to tell and its especially easy to check which version of similar words is more common like 悪癖 and 悪い癖. For novels its a lot harder to tell which words are actually the extremely rare ones based off natives because some natives just don’t read as much I guess? From different natives, I was told they didn’t know 千仞 and 諦観 (xd?) respectively. One of those is extremely rare, the other one isn’t and the native who said that probably just doesn’t read many books. The frequency dictionary helps make this a lot more apparent.
Its not perfect and will have its own bias, but is overall a positive addition to yomichan for anyone worried about how often words are actually used in some setting.
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I’ve seen it used with the second meaning a few times, which makes me think it’s not super rare.
This example from Jojo: 博打を避け戦いの駒を一手一手動かすそれが真の戦闘だ
Of course in anime everything goes, so you should be prepared to learn the rare stuff as well.
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I think it’s extremely useful to make a distinction between words that are rare in conversation and words that are rare in writing. 博打 is not how you’d say it in everyday conversation, but the word is common in texts. Same applies to 欠かす.
The distinction exists in virtually every language with a written form, and it’s why I hesitate, or even oppose, labelling words rare in everyday conversation as “useless”. Written language is almost universally conservative, and terms that may seem too outdated or old-fashioned for everyday use flourish and prosper in that environment.
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The Japanese people I have talked to are very familiar with written Japanese. They still haven’t heard of 干天.
Well, then, unfortunately, since the explosive popularity of Demon Slayer they are now less literate than literally millions of Japanese elementary school student who know about Tanjiro fifth’s form 干天の慈雨
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So you know they are people who read a lot of prose? So far unfortunately almost none of the Japanese I’ve met have been into books. It’s easy to find sample sentences where it is used, although it does seem uncommon.
Nevertheless, any Japanese would instantly know the meaning and know how to read it, even if they had not seen it before.
Works fine in WK to enforce the reading, even if it’s not a word used in conversation.
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I agree a lot with this, honestly. Sometimes the rarity of a word can also be affected by how often one really talks about topics related to said word.
Like today I came across 縄梯子 (rope ladder). Not a common word according to Jisho with very little info altogether. However, possibly extremely useful, considering rope ladders are involved in rescue scenarios. Fortunately, can be inferred from 縄 and 梯子 so remembering it alone is not super necessary.
Well, I guess also in the mountains when hiking along steeper routes.
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The only way to get an idea of the most common Japanese way to express a concept is to engage with more Japanese. The learners on this website certainly are not going to be able to help you, and frankly it’s a hard question for native speakers to answer as well because they don’t think much about these things. Instead of fretting about this, just focus on learning and engaging with native content and you’ll find the correct words coming to your tongue naturally (they will be the ones you’ve heard a million times on TV and so on).
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I have no opinion, and I’m not qualified to have one. I’m just saying what she said.
But the context matters - she was teaching a bunch of American military clowns a very basic Japanese class. I was miles above these guys with my meager N5 grammar and WaniKani vocabulary. She often said things like “that’s grammatically correct, but not how we would say it” or “that sounds way too formal” (in the context of, clueless American trying to find something in a grocery store). She might not have been saying nobody ever uses that verb, just saying that’s not how people would say the thing you’re trying to say, overall.
But this one in particular, she had a puzzled look like she legit didn’t even recognize what actual word I was trying to say, until I wrote it down. For whatever that anecdote is worth. I would say I’m avoiding that word in favor of another one, but I can’t because my vocabulary isn’t big enough to have that many choices.
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