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]
A member of the Youth International Party, a left-wing group espousing values of the counterculture movement of the 1960s and early '70s
[1969+; fr the acronym of the name of the group, reinforced by the rhyme with hippie]