It does have the connotation of someone who gets wasted and parties while attending these festivals, but the overall meaning is based on simply frequenting festivals.
"So you said Steve might be coming to forest this year?"
"Yeah, but I don't think it's gonna happen, he's not much of a festie."
when two fat people are sitting one seat apart in the movie theater and their respective fats reach across the empty seat and touch throughout the movie.
similar to footsie.
the guy sitting in the seat beside the seat i was sitting by was fat too and our fat touched. we played fatsie the whole movie.