I always thought the reading for 勢い (いきおい) was to strange to be ‘just an exception’, which seems implied when the WK reading mnemonic begins “This is a rough one. …”. So, I went digging in Jisho.org to see if I could find any kind of ‘etymological’ clues for where the word comes from. I often find looking for the ‘etymology’ of words to be useful for remembering them (whether for meaning, or for reading, or both) – even if the ‘etymology’ I ‘discover’ is really just some faux-etymology I came up with. Jisho isn’t really intended as an etymological dictionary after all; it just has a quite decent search engine, allowing to search for connections between words in a variety of ways. But anyway, I digress.
In the end, I did not find any useful ‘etymological’ information for 勢い, which again actually struck me as quite weird. Usually there’s something to find.
Regardless, I ended up coming up with another faux-etymology which – even though I’m only going off of my intuition and nothing official/researched – I think is actually pretty darn good. Just have to consider it as strictly a mnemonic tool, and take it with a grain of salt. Still, I think it would serve well as a mnemonic. Hence I’m suggesting it for WK, if it might help supplement the existing reading mnemonic (which. being a little frank, I didn’t really find very useful )
It’s a bit long and pedantic, but could surely be condensed and WaniKani-fied to a shorter version. Anyway, here it is:
Sketch of the Mnemonic: