'Slag' (dog,devious,untrustworthy,grass) 'Boat' (face)
A slaggy boat. Somebody considered untrustworthy, reguarly pulling snakey moves on friends and just generally being a c*nt. Hangs about with gangster crowd pretending to be a drug dealer type but never sold a penny's worth.
That paul's a right slagboat - I've got no time for the geezer.
John, you cheeky fu*king slagboat wheres my puff?!?