A word invented for the dance floors of concerts (mainly Ska or Punk) where there are just to many people to get a half decent mosh or skank pit going. Also when there's pretty much no room to move, but people still manage to thrash around, and if one person falls to the side they manage to bring down that half of the floor with them.
the Reel Big Fish and Streetlight manifesto show had one hell of a suck pit
Guy#1-dude, what the hell, this skank pit is lame!
Guy#2-it's not a skank pit, its a suck pit!