When a person gets bored of one persons vagina so has to swap from fanny to fanny. Also if a person does not get action from his girlfriend or only once or twice a week so finds other girls so he can get gash all week.
Joe: Woww i've got a proper gash mash-up this week lillie on monday then ellie tuesday then my girlfriends sleeping over wednesday but friday i think i will see Georgie.
Jack: Ahh cant be arsed my girlfriends fanny is to tight i get bored might as well go fuck Georgie.