a member of a group of radical, politically active hippies.
Origin of yippie
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
1968, acronym from fictitious "Youth International Party," modeled on hippie.
On December 31, 1967, Abbie [Hoffman], Jerry [Rubin], Paul Krassner, Dick Gregory, and friends decided to pronounce themselves the Yippies. (The name came first, then the acronym that would satisfy literal-minded reporters: Youth International Party.) [Todd Gitlin," The Sixties," 1987, p.235]
Online Etymology Dictionary, © 2010 Douglas Harper