It is an acronym for Beat n Leave which means to have sex with a girl and have no contact with her after or vice versa. It was first used in 2009 when Benjie (founder of BnL) said to friend 'she's average would have to bnl dat still thou'
Person one: Why you wid dat chick
Person two: what you mean?
Person one: You should've just bnl she's a 5
Person two: I know man its peak.