A long-haired 60s flower child was a “hippie.” “Hippy” is an adjective describing someone with wide hips. The IE is not caused by a Y changing to IE in the plural as in “puppy” and “puppies.” It is rather a dismissive diminutive, invented by older, more sophisticated hipsters looking down on the new kids as mere “hippies.” Confusing these two is definitely unhip.
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