Shortened form of "definetly possibly". Since anyone that says "definetly possibly" sounds like a complete fool, someone came up with "def poss". Doesn't really save much time in speech, but it does save a person's reputation. The advantage of saying "def poss" as opposed to "possibly" or even "poss" for that matter is the fact that "def poss" implies that is a serious possibility, rather than someone just teasing with a half-hearted "possibly".
John: I just got a new set of kicks
Bill: Those are def poss the coolest things I've ever seen.
John: Wow, what a chill broseph
you are for saying def poss.