Definition of jackknife in English:



  • 1A knife with a folding blade.

    • ‘With a jackknife, he whittled a point on a thin green stick pulled from a maple branch.’
    • ‘The shorter girl snarled her frustration and lunged after him with her jackknife, tearing a horizontal gash in his right pant leg.’
    • ‘Pen clips, protractors, and jackknives have left hacked shapes to tell people how dull it all was when they sat here.’
    • ‘Adam had chosen a jackknife rather than a fixed blade.’
    • ‘I ducked down in onto my hands and knees, and right before my eyes, I saw the jackknife that Todd was carrying discarded on the ground, it was tremendously shiny, and glittered in the company of a red handle.’
  • 2A dive in which the body is first bent at the waist and then straightened.

    • ‘Westmount borough chairman Karin Marks, on the threat of non-residents coming en masse to do jackknives in the Westmount pool’
    • ‘With his opponent flat in the center of the ring, Van Dam climbs to the top turnbuckle, springs across the ring crouching and extending his torso like a diver performing a jackknife.’
    • ‘Though his past leaps of faith have had about as much grace as a jackknife into a leaf-covered pool, bringing Gibbs back has ‘reverse pike triple somersault’ written all over it.’
    • ‘But still, I usually like to lounge about in a chaise lounge or something while everyone else is doing cannonballs and jackknives and freakish flips off the diving board.’
  • 3Statistics
    A method of assessing the variability of data by repeating a calculation on the sets of data obtained by removing one value from the complete set.

    • ‘We used the jackknife version of cross-validation, where the training set consists of the complete network minus a single specified edge.’
    • ‘To evaluate clade support, bootstrap and jackknife analyses were performed in PAUP * using a full heuristic search with 1000 replicates, simple addition sequence, and TBR branch-swapping.’
    • ‘Higher-order jackknife estimates lead to successively greater reductions in the bias of the estimates of N d, but at the cost of increasing sampling variance.’
    • ‘Bootstrap and jackknife analyses were performed under the MP criterion, with equal weights for all positions and two random additions of sequences for 200 replicate searches.’
    • ‘Occasionally, low Bremer support is correlated with a high jackknife percentage, e.g., at the node separating the Wnt1 subfamily from the rest of the tree and at the base of the vertebrate Wnt 10 sequences.’


  • 1 Move (one's body) into a bent or doubled-up position.

    ‘the Major jackknifed his thin body at the waist’
    [no object] ‘she jackknifed into a sitting position’
    • ‘Galindo devilishly liked to stun nurses by jackknifing himself into a completely folded hospital bed.’
    • ‘Jimjim's body suddenly jackknifed, wrenching his figure to the side.’
    • ‘A spasm jackknifed me, crunched the air right out of me.’
    • ‘Students then jackknife their legs even closer to their bodies to the rhythm of a driving beat.’
    • ‘With a harsh indrawn breath, I jackknife to a sitting position.’
    1. 1.1[no object](of an articulated vehicle) bend into a V-shape in an uncontrolled skidding movement.
      • ‘Mr William tried to slow down by pumping his brakes but the vehicle jackknifed in the road while trying to avoid Mr Utting.’
      • ‘The snow came down in handfuls and cars jackknifed to the left and right of us as we drove east on unplowed streets.’
      • ‘A semi-tractor and trailer jackknifed coming down the icy hill on Northeast North Woodinville Way at about the 14500 block.’
      • ‘A double-semi had jackknifed, neatly shutting down four lanes.’
      • ‘Similarly, neither of the two units will be damaged if the trailer jackknifes with respect to the tractor.’
    2. 1.2[no object](of a diver) perform a jackknife.