Bastardised version of cashback. Used to describe the taking out of cash in order to go out and chase gash
Gentleman: Two cartons of um bongo and a packet of skittles please shopkeeper
Shopkeeper: Certainly sir, any gashback?
Gentleman: £20 please, I fancy a spot of wenching tonight and need some funds
Shopkeeper: Very good sir
something or (someone) that is so completely and utterly gash, they qualify for gash back. in short, gash of epic proportions. or just generally a term for skanks, who spend the majority of their time on their backs.
you're more than a slut, you're a walking gash back.