Someone who is an idoit, a wanna-be hoodlum, a bitch, a slut, whore, tramp, player, etc. or anyother insulting and degrading term. It also my be used as a term of endearment.
joe: "yo! look at my new pants, these jaunts can wrap around my entire body twice!"
john: " dude, your such a slum duppy."
--or--
jane: "oh my gosh your always forgetting about our plans!"
bob; "your such a slum duppy."