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1935, American English, probably from icky-boo (c.1920) "sickly, nauseated," probably baby talk elaboration of sick. Originally a swing lover's term for more sentimental jazz music; in general use from 1938.
A conventional, tedious person; square: She turned out to be an icky (1935+)
[fr baby talk, ''sticky, nasty''; although perhaps fr Yiddish elken or iklen, ''to nauseate, revolt'']