1.Used to describe a woman who you can have potential sex with but you can't screw her at the moment because she has a boyfriend, or lives far away, etc. so you put her on "standby" while you go have fun with some other females.
2. Basically a rebound girl.
Bruce: "Ey Ozzy, did you screw that mexican chick yet?"
Ozzy:"No bro, she's "standby pussy"; she's got a boyfriend, I'm gonna fuck her later and in the meantime I'm gonna get at these other bitches!!!"