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To be someone's backup if their other, more desirable option falls through/passes out/falls off a balcony/runs out of money.
A: "What are you up to tomozzle?"

M: "I'm going out with the ladies to try and pick up. But I'll call you if nothing comes through"

A: "Negative Dude, I'm nobodys Second Choice Susan, forgit it"
by Andrew McUtchen October 11, 2007
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