Douglas Harper's Etymology Dictionary
also jackknife, large pocket knife, 1711, probably American English, "perh[aps] associated with some sense of JACK sb.1, but compare jackleg knife" [OED]; see jack + knife (n.). Jackleg was a U.S. colloquial term of contempt from c.1850. On another theory, so called because it originally was associated with sailors. As a kind of swimming dive, from 1922. As a type of tractor-trailer accident, 1966. Both from the notion of folding, as the knife does.
1776, "to stab," from jack-knife (n.). Intransitive meaning "to fold or bend" the body is said to date from the time of the American Civil War. The truck accident verbal sense is from 1949. Related: Jackknifed; jackknifing.
Wiktionary
alt. 1 A compact folding knife. 2 The front-dive pike, in which the body folds and unfolds. 3 (context colloquial English) A semi-trailer truck accident in which the vehicle mimics the closing of a jack-knife. 4 (context statistics English) (alternative spelling of jackknife English) n. 1 A compact folding knife. 2 The front-dive pike, in which the body folds and unfolds. 3 (context colloquial English) A semi-trailer truck accident in which the vehicle mimics the closing of a jack-knife. 4 (context statistics English) (alternative spelling of jackknife English) vb. 1 To fold in the middle, as a jackknife does. 2 (context colloquial English) To cause a semi-trailer truck to fold like a jackknife in a traffic accident.
Usage examples of "jack-knife".
Lying awake in the night, with the jack-knife ready to his hand, he had decided on the course of action that must be adopted.
Then a horrible prompting--arising out of his former cowardice-- made him feel for the jack-knife with which one murder had already been committed.
In regular bounds the white haired girl vaulted and spun against the sky: back triple somersault, front triple somersault, half gainer, recovery, full gainer, recovery, jack-knife opening backwards in a reverse swan, triple back, then triple front again.
The muzzle of the Hanyatti caught him just under the breast-bone and I stood aside as he jack-knifed and fell to the floor.
Even as Smith watched, Carraciola seemed to rise effortlessly three feet into the air -- a feat of levitation directly attributable to the powerful boost given him by Thomas and Christiansen -- jack-knifed forward at hip level and flattened his body on top of the cable-car roof: his legs.
He made no sound at all, just jack-knifed forward in turn as convulsively as I had done, fell against me and slithered slowly to the floor.
One leaned on the pig’s rump in case she jack-knifed, another pulled back her head by those lolling ears.
He followed them with caterpillars that twisted and jack-knifed as they fell towards the paved ground.
As Isaac and Yagharek watched, it jack-knifed at its centre and seemed to kiss its own tail end, slowly relaxing until it hung deadweight again.
It sent waves of transforming current through thickly insulated cables, towards Andrej, who rolled and jack-knifed in spastic terror and pain.
Palms and back against one side of the chimney and stocking-soled feet against the other, Mallory jack-knifed his way upwards until the widening sheer of the walls defeated him.
For two or three seconds the Oberleutnant stood motionless in his tracks, his face shocked and contorted: then he jack-knifed violently forward and pitched down into the crusted snow.
Deep inside him he knew that he had no chance in the world, that he couldn't hold Coco off for more than a few seconds, but he told himself that if ever he was to have a chance it was now, before the fight had started, while the element of surprise still existed as a possibility, and even with the thought he was hurling himself across the room, his legs jack-knifing open as his feet reached for the giant's chest.
Once down, it seemed the safe and prudent thing to do to keep my centre of gravity as low as possible by staying there, and I made it to the top of the stairs by jack-knifing upwards on the seat of my pants and the soles of my shoes.
Isaac could see the fat grub wriggling, jack-knifing in excitement, twisting desperately against the wire front, squirming with sudden reserves of energy towards Lucky Gazid.