What is the X in X-mas?
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]
An exclamation of dismay, alarm, emphatic response, etc: Yipes, it's a rattlesnake! Yipe, that hurt!
[esp teenagers; probably fr the spontaneous interjection yi, a cry of pain or dismay, with the p or k stop intruding after lip closure, as it does in nope and yep]