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1.
When a man has an orgasm while his penis is semi-erect.
"Man, I was so sexed-up last night - I started jacknifing the second she put her lips on my cock".
by Wobberino September 22, 2005
 
2.
when you bend a dog in half and stick your knob in its mouth while someone else does it in the poopchute with a strap on. This way you and the other person can rub rusty sherrifs badges.
I jacknifed my collie last night. I think he might of liked it.
by dangerous dave April 13, 2005