Jackknife

v. bend like a pocketknife, double up, bend sharply in half; do a jackknife dive

n. large pocketknife whose blade folds into the handle; dive in which the diver doubles up in mid-air and then straightens out before entering the water

Jackknife may refer to:Jackknife (truck tractor and trailer), the folding of an articulated vehicleA pocket knifeJacknife, a film starring Robert De Niro and Ed HarrisA professional wrestling holdA martial arts kickA type of dive in underwater divingA resampling statistical technique
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Noun
1. a large knife with one or more folding blades
(synonym) clasp knife
(hypernym) pocketknife, pocket knife
2. a dive in which the diver bends to touch the ankles before straightening out
(hypernym) dive, diving
Verb
1. dive into the water bending the body at the waist at a right angle, like a jackknife
(hypernym) dive
(classification) sport, athletics

(n.)
A large, strong clasp knife for the pocket; a pocket knife.
  

(See also Accident, Rollover) Jackknife can occur at any time during the crash sequence. In this report, jackknifing is restricted to truck tractors pulling a trailing unit in which the trailing unit and the pulling vehicle rotate with respect to each other. (NHTSA3)