Someone who is very sexy and pretty / handsome and is believed to be worthy of having sex with or 'banged' which resulted in the slang word 'bangable'. A person being called 'bangable' is therefore being told that the person want's to have sexual intercourse with them or 'bang' them.
*girl walk's into bar who is very beautiful*
man1: "wow, dude, look at that, she's fucking bangable!"
Man2: "I know dude, I'd bang her any day of the week!"