Top Definition
The act of multiple men sodomising eachother in a human train. Just before ejaculating each one pulls out and blows their load all over the back of the guy in front shouting "twenty-one !"
Damien the sick fuck went out and played backjack with 5 guys he met at a gay bar.
by DamienN July 13, 2006

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The act of jacking off your boyfriend whist he is sat behind you as you are typing this onto his laptop at the same time. During this he is sitting on his Nintendo DS playing Pokemon.
Example: having a laptop on your knee; sitting on the end of the bed; penis in hand! There you have, a backjack!
by Carrots. June 29, 2014

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Verb.

the act of paying someone back in cigarettes for a cigarette bummed earlier.
catherine: hey john, can i bum a cigarette?

john: sure, as long as you backjack me
by BOBBLE BOO October 19, 2007

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A form of masturbation where you slide your hand down your dick with your hand backwards, forefinger and thumb touching eachother. Your thumb should be facing in twards your balls and your pinky out twards the head.
Painting a face on your hand is optional.
"Sometimes I rub one out backjack style to shake things up a bit."
by Clam Choder April 25, 2007

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the art of bending the penis through the back of the legs for a handjob
she said she was gonna show me something ive never seen before so she pulled my pp through my legs and gave me a backjack
by peepsey March 23, 2008

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