A member of a group of politically active hippies, originally in the US.The hippies and yahoos and yippies and SDS students were seen as a real threat to the fabric of society....
I remember when thousand of hippies and yippies surrounded the Pentagon during the Vietnam War and held an exorcism to drive out the evil demons.
Holly screamed with delight and Molly let out a yippie.
Origin
1960s: acronym from Youth International Party + the suffix -ie, suggested by hippy1.