It doesn't stand for anything. A slab is a synonym for slut. Reffering to always being on her back like a slab of wood or cement. Popular among the urban population but slowly making its way around.
Did you hear about what stephanie did last night? She gave a birthday blowjob to a random guy then went and fucked her boyfriend. What a slab!
by Slabbyslab August 2, 2011
Get the Slabmug. by p3anut August 5, 2008
Get the slabmug. Down south we ride slabs
by J-HTX July 14, 2007
Get the slabsmug. by The Krunk Wizard Giz August 9, 2006
Get the slabmug. 

