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.
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