While certainly checking a monolingual dictionary would be ideal, ごりごり can also be found in J-E dictionaries:
Jisho: ごりごり - Jisho.org
The Jaded Network: Define gori gori | ゴリゴリ | ごりごり - Japanese-to-English SFX Sound Effects Translations @ The JADED Network
onomatopedia.jp can also help sometimes, though it’s more limited. In this case it doesn’t have ごりごり.
What’s the difference between 頭 and 脳? I’m used to 頭 meaning brains/smarts because of the line 魚を食べると頭がよくなる (if you eat fish, your head will get good/you’ll become smart) (歌：おサカナ天国), but then there’s 脳 which means brain(s). So what’s the difference/nuance? Was 頭 picked just because it has the same number of mora as 魚?
頭 = head, 脳 = brains. 頭がいい means “smart” in the same way as “(he) has a good head on (his) shoulders” does in English.
I wouldn’t say in English that it’s possible to get a “good head” in that sense though. I’ve always heard it used for people who have good reasoning abilities rather than people who might just be book smart. 
If you want to emphasize that someone is book smart and nothing else there is 学者バカ, but it’s a pejorative expression.
Otherwise, Belthazar is right.
I said that more as an analogy rather than direct contrast. My point is, 頭がいい is an existing phrase in Japanese, and (as any mother/grandmother anywhere will tell you) eating fish makes you smart. There’s no deliberate wordplay going on here.
Ok, so would fair to say then that they both carry the same meaning of smarts and that smarts aren’t really as divided in Japanese as in English (book vs street/reasoning) then? And that 頭がいい is just a preferred expression?
脳 is literally the brain, the actual object in your head that does stuff. I think it’s possible for 脳 to mean “brains” as in “smarts” but first and foremost it is a physical organ.
In that case, does this example sentence from WK for 脳 imply that the person in question is stupid or that they might have some sort of mental handicap?
アイツは本当に脳が弱いよな。
Just to make things clear.
Both 頭 and 脳 exist independently from their phrasal use and are completely different words. Also, none of them means “intelligence” or “smartness” per se. These are secondary meanings that will only appear when used in a figurative way with proper adjectives or verbs following.
頭：Head. Where not only you brain is, but your face, ears, throat, hair, etc are too. It’s also connected to your neck, has bones inside, etc.
脳：Brain. A specific organ that happens to be placed inside your head. It has several functions, has neurons, it’s soft, etc.
That’s why there are countless sentences that can only be said with one or another.
この帽子は私の頭に小さすぎる。
彼は脳外科の先生です。
先ほど頭をぶつけてめっちゃ痛い。
脳は神経系の中枢である。
However, regarding their figurative/phrasal use, they can indeed be very close, since both are related to thinking skills in a broader sense. That’s why replacing 頭 for 脳 in many figurative sentences works just fine.
Japanese as a language has as many ways of saying one is smart/intelligent one could expect. 頭がいい and 学者バカ are two of them. 学力のある、頭の回転が速い、賢い、ずる賢い、知的、知性のある、洞察力のある、察しの良い、知識豊富 are some of the countless other ways of saying it, each with different nuances of book/street smart, reasoning, knowledgeable, etc.
In that sense 頭がいい is one the broadest and simplest ones, being possible to use it in both suggested contexts:
彼が本当に頭良くて何でも察せる。気を付けたほうがいいよ。
また満点取れたの？どうやってそんな頭よくなれるか教えて、マジで。
脳がいい is not a set phrase as 頭がいい is. But since the figurative meaning is pretty much the same, on most situations Japanese people would follow it without a problem.
Also, I do not think 頭 and 脳 have a specific nuance difference when used in this way. If anything, 脳 is a “fancier” word than 頭がいい, which is elementary school level vocabulary, so maybe you could use it as emphasis.
About the 「アイツは本当に脳が弱いよな」 example, I do not think it necessarily means an actual disability/disorder. Here too brain is being used in it’s figurative sense and could be replaced with 頭 if so desired. For instance, when talking with a 6 years old, I think any Japanese adult would replace it without a doubt.
Also, 弱い is a pretty weak (no pun intended) and common use word. If you wanted to imply an actual disorder you would need to be a lot more specific. With this wording you are just saying you find them… “not really gifted”.
Thank you for your fantastic explanation! This is exactly what I was trying to figure out.
In Aggretsuko s03, there is a stalker character ワツペソ男
https://aggretsuko.fandom.com/wiki/Unhinged_Fan
What does ワツペソ watsupeso mean?? Unhinged? It’s just, I falied to find it up online/jisho.
The wiki appears to be incorrectly transcribed from the subtitles - it’s not ワツペソ男 but ワッペン男, presumably because of how he’s introduced:
(ワッペン means sewn-on badge like the one he has there so it’s clarifying who’s talking for the hearing-impaired by calling him “badge man” basically)
Per the credits:
a plan is what’s scheduled to happen in the future - 予定 is like “fixed beforehand” - you fixed beforehand your plan for the future.
a hypothesis is what you think is going on but might not turn out to be the case in the end - 仮定 is like “temporarily fixed” - a hypothesis is temporarily decided on, but its veracity is pending the result of the experiment.
Example sentences from Weblio maybe show the difference:
旅行は来月に予定している - like “I’m planning to travel next month.” - the trip is a scheduled plan.
今ここにコップがあると仮定してみよう - like “let’s say there’s a cup here” - they’re supposing a cup is there for the sake of whatever argument they’re making but the hypothetical cup won’t survive the scope of the argument since it isn’t really there. Hence it’s 仮に定めた事柄。(this sense is the other sense of the word along with hypothesis)
You mean you are looking for a way to remember that 予定 means plan/schedule and 仮定 means assumption/hypothesis ?
The kanji are fairly telling I think.
定 means determine or decide
予 is beforehand. So 予定 is something decided in advance like a plan or a schedule
仮 is temporary. So 仮定 is something you “temporarily decide”, so tentatively determine for now, like an assumption or an hypothesis, but you are not sure it’s true yet.
yes
yeah I get it now thanks
So I looked おもむろに on jisho and it came up with two definitions that are pretty much the opposite of one another:
Are both of these used? Please tell me that’s not the case 
Edit: Just to ask everything at once, this is the sentence I was reading:
叔父は懐からおもむろにタバコを取りだすと、手で風を遮りながら、百円ライターで火をつけた。
Assuming both definitions for おもむろに are equally used, did he take out the cigarretes abruptly or slowly?
Not that it matters greatly, but I’m curious how someone with more reading experience would interpret that.
I looked around a bit at weblio, and it sounds like the “deliberately; slowly; gently”/ゆっくり meaning is the correct one, but people have come to mistakenly think it means the “suddenly/abruptly” to the point that it merits being listed in some dictionaries - I suppose like a word like say, “inflammable” in English. Oh, or “nonplussed.”
This article seems like it has more information:
It sounds like according to that article at least, the misunderstanding arises from the same reason you’re having trouble with the line - it’s just not something you can tell from context, and either meaning makes as much sense and it’s unlikely to make a big impact so people just often come away with different impressions.
One of the Weblio definitions has commentary mentioning a 2014 poll suggesting 44.5% of people use it to mean ゆっくりと and 40.8% 不意に, so it seems pretty mixed but I would go with the slowly version since it seems to be the most correct option.
Context is everything.
It’s not unheard of for words to mean opposite things, especially when slang gets involved. やばい for instance can mean something’s really bad or really good. English does it too - “literally” means both “literally” and “figuratively”. “Bad” can be used to express something’s really good.
On its own it’s impossible to say what the author intended. Context will make it a lot clearer. In this particular case, taking out some cigarettes is not something you’d usually do abruptly or suddenly. If it’s a natural action to perform in context and nothing indicates anyone’s surprised, I’d say it probably means “slowly” here.
I’d be careful with conclusions like these. “Officially” it may be the more correct option, but that doesn’t mean other meanings are any less valid, especially in a colloquial context.
