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1.
To figure out your chances of having sex with your partner or on the first date.
Guy #1: Dude did you nail her.

Guy #2: Not yet I'm still summing
by T.I.T.O May 15, 2009
 
2.
Sport or recreation
Bob: "Do you like summing?"
Jake: "Ya, I play baseball all the time"
by Isabellathefirst January 13, 2009