Over dinner recently, the Far Outliers were talking about ways to describe food with a visiting Japanese college student (the niece of old friends) here for a bit of English immersion. The only Japanese equivalent for gross or yucky that came readily to mind was kimochi warui ‘unpleasant feeling’, which she shortened to kimoi.
(“Kimochi warui” is how my feistiest niece’s kindergarten classmates in Japan described her blue Irish eyes. “Your blue eyes give me the willies!” She always fought back when teased or excluded—and still does to this day.)
chi warui ‘feeling bad/unpleasant’
o kusai ‘troublesome’ (lit. ‘stinking of trouble’)
shiroi ‘interesting, funny’ (written ‘whitefaced’)
I don’t think you can describe these shortenings in strictly mechanical terms, especially when you include the final member of the set: uzai for urusai ‘noisy, aggressive, bossy’. (“Urusai!” is what people yell at loud revellers to tell them to pipe down.) In the other cases, the shortening rule seems to be to keep just enough syllables (or moras) to turn the compound into what sounds like a one-word adjective ending in -i in the present tense and -katta in the past. I suspect omoroshiroi ‘interesting’ would have ended up omoi were it not for the inconvenient homophone omoi ‘heavy’.
rashii (‘strange, curious’), ne?
UPDATE: Two commenters who are far more kuwa
shii than me on Japanese have suggested that uzai is most likely short for uza ttai, not urusai. That makes the shortening look a little more regular.