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1. In baseball, when two consecutive batters each hit a home run.
2. The act is sitting back-to-back with someone, while giving each other reach-around manual stimulation.
1. "Dude, I went to the Orioles game last night and Roberts and Markakis hit back-to-back jacks!"
"What was the final score?"
"They lost 10-2."

2. Tom and Kevin had some drunken back-to-back jacks last night, claiming it's not gay if they can't see what they're doing
by Castrati July 24, 2011
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