wally
An insult that became popular in the 80's, meaning an idiot.  Scholars of urban slang have long believed this word originated at Woodstock (15-17th Aug'69), when some hippy lost his dog called Wally.  He wandered around the site shouting Wally, and before long the crowd started shouting it.  After that, wally became a popular chant at music festivals.  However, on a recording of a Jimi Hendrix concert on 9th Jan 1969 in Sweden (7 months before Woodstock), someone in the audience shouts wally while Jimi is dedicating a song, prompting him to say "yeah, this song is for wally too".  So, while the word probably did originate at a festival, if has to be another one, not Woodstock.
"You are a complete wally."