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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 26, 2005
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When a trailer being hauled loses traction and begins sliding to the side of the vehicle that is hauling the trailer. Similar to a rear wheel drive vehicle fish tailing and losing control.
Passenger: Dude you better slow down on that turn, its pretty icy out there.

Truck Driver: You're right, I wouldn't want to jack knife and cause a wreck.
by afrojim2009 June 14, 2010
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a semitrailer truck accident where the truck portion folds backwards so that it resembles a jack-knife
I realized I had to take a detour when I heard about the delay caused to a jack-knife on the freeway.
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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 November 29, 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 November 24, 2007
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Having deposited your turd in the toilet, you look back to discover that its poking out of the water, too long and solid to go round the u-bend.
I had to go and fetch a stick so I could break up the jack-knife and flush the little devil.
by n_m April 01, 2004
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