BEG SLAG. Share definition Flag A person who lies about sleeping with their mates boyfriends to fuck up their relationship because they are a beg scene and has no hair. That girl Alice is such a beg and A SLAG so she's a BEG SLAG. BEG SLAG. by Lanaloo January 18, 2008 0 0 Get merch Get the BEG SLAG. mug. Buy Now — $32.95 See more merch