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2.
A girl that a guy keeps on the side (or in his ‘back pocket’) for when things in his current relationship go badly or end all together
When his new wife decides to throw bitch fits or is holding out on him, Mike calls on his Back pocket girl, to suppliment him, but totally ignores her otherwise.
by backpocketed January 19, 2011
 
1.
A girl that you keep around for company when your bored or loney and in-between girlfriends. Also called a surrogate girlfriend.
John: Hey who was that you were on a date with last night? I thought you and Marie were together?

Michael: Nah man...she's just my back pocket girl till I find someone I wanna settle down with.
by hollypete September 07, 2009