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1590s, "resembling a peach" in some way, from peach (n.) + -y (2). Slang sense of "attractive" attested by 1900. Extended form peachy-keen recorded from 1953. Related: Peachiness.
Excellent; wonderful; great, neat: this president's political health, which Wirthlin thinks is peachy (first form 1900+, second 1955+)