Top Definition
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".
#pre-ejaculae #pre-cum #chubby #semi-wood #flaccid.
by Wobberino September 22, 2005
When a hammock is set up taller than it is wide.
Wow, that person is jacknifing in their hammock like no one's business.

I mean, I've seen people jacknifing at a 90 degree angle before, but that person is jacknifing at an impressive 20 degrees.
by katgumball May 23, 2016
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
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