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From what I know, it's when an 18-wheeler is carrying one of those loads, any loads, as long as it's carry a load, and it's going at quite a highspeed, suddenly braking, causing the actual truck itself to brake whilst the load's momentum behind the truck forces itself in motion against the truck, causing the load on the back to be intersecting at a perpendicular angle with the truck. Look at this---> |_ The | is the actual truck and the _ is it's load. The truck (|) is going forward, whilst the load (_) is scraping it's tires horizontally.

A jackknife is also responsible for causing many highway accidents, because when a truck jackknifes, it takes up all the lanes on the highways, and leaves no mercy for the vehicles in its path! Always speed up if you see a jackknife coming toward you if you cannot rapidly get off the highway.
*people driving*
Oh crap dude, that truck is in the process of a jackknife, that piece of shit volkswagon is in our way, guess the only choice we have is to speed up, step on it.
*person in volkswagon*
*ahhhh this is the life, *fart*, beer and hot sauce...boomheadshot.*
by Cobain will have justice. November 28, 2005
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1. n.-a pocket knife

2. n., v.-person or object that is bent unusually at a 90 degree angle resembling the shapre of an open pocket knife, as in the case of a car accident or physical movements used in disciplines such as diving, gymnastics, yoga, wrestling, etc.
"That diver has incredible technique! Note how he jackknifes his body to move into that maneuver in mid-air."

"Oh, shit! Did you see that car accident? It looked like that tractor-trailer was totally jackknifed!"
by Lapa December 28, 2005
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sex move in which woman on her side bottom leg straight top leg bent up to her chest
i slammed her Jackknife style!
by JBut December 25, 2007
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A "Jackknife" is made by dropping a shot of Jack Daniels into a glass of Corona, depth charge style.

Similar to a Jägerbomb.
Bar Guy 1: Just give me a beer
Bartender: And for your friend?
Bar Guy 2: Ill have a Jackknife, cause I'm a real man.
by Muhammed Ali Ganoo October 20, 2008
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The action of pulling your pants down while driving, squatting above the seat, reaching your hand underneath your bottom, defecating into your hand, and then using the feces to smear your windshield. Done ritualistically for courting purposes.
The man decided to jackknife while driving on highway 99, in an attempt to enamor his lady friend.

There are people who jackknife to relieve stress during bumper-to-bumper traffic.
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A knife that folds out of the handle!

You know you’re dead when that jackknife folds!
Me: *sees jackknife*
Stabber: *folds knife out of handle discreetly*
Me: *runs away*
by Stabberman420 March 31, 2020
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When you jump off of something and the front of your foot lands on something but not the back, causing your foot to be shoved upward as you leg goes down
“I just jackknifed my foot on the stairs, my Achilles tendon is torn”
by Onlygirls5'10andabove January 28, 2024
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