As an adjective: If somethin' is pimpin', it's pretty darn cool. It's probably something "normal" that's tricked out ghettolicious and gawdy.
Basically, you look very ghettofab and blingbling.
However, as a verb
1.) to pimp something out is to *make* it look very ghettofab and all that nifty stuff in the above paragraph.
2.) to pimp is to advertise (generally, in an enthusiastic sense) or to call attention in order to bring acclaim to something; to promote.
Somebody buys some hideous plaid pants and then adds about five pounds of metal chains ... well, that's pimpin'.
A friend creates a collage of ... Angelina Jolie's various tattoos ... you pimp the collage to others, so that they'll see her work.