LB being the abbreviation for pound (in the case referring to rough sex ie: she got pounded) and Cakes short for Pink Pancakes as defined on this site. Thus LB CAKES = Pounding out a females private area.
Guy 1: Wat did you do last night?
Guy 2: Son I met this cute ass red bone chick!
Guy 1: Word?! Did you LB CAKES
Guy 2: Nah, her nappy headed friend was cock blockin!