yucky!

haha..i saw this when i picked up my sister from the tutoring place she attends. i am aware that "trash p/u" could possible stand for "trash pick up" but i'm going to pretend that whoever wrote this knew that the trash was going to smell terrible, as it stereotypically should. hehe, i thought this was all in fun. but i have a question...
where does the phrase "p.u.!" (maybe i'm spelling it wrong) come from?