A person, usually one of the female variety, who comes off as a major slut at a prom party by either:
1) Wearing a very skimpy, inappropriate dress
2) Dancing in a very sexual manner with partner
3) Stealing other partners
4) Fucking around during prom, or
5) All of the above.
Girl: Oh my god, look at that girl, she is dressed like a total promstitute.
Boy: Oh dude, did you totally see that promstitute dancing in the techno room? I'd totally tap that!