Acronym for an Urban Dictionary Certified Professional. Basically, one who is lucky enough and/or excels at getting entries accepted by Urban Dictionary.
Steven, your GISP is meaningless when compared to the accomplishments of Garth. I mean, dude, stop including it in every e-mail you send me. Garth is a true scholar as proven by his UDCP credentials. It even trumps Kyle's MBA and Nintendo DS masters.