Simple Definition of jackknife

from our Learner's Dictionary

jackknife

1 of 2

noun

plural jackknives
1
: a small knife that has a folding blade : pocketknife
2
: a dive in which you bend from the waist and touch your ankles and then straighten out

jackknife

2 of 2

verb

jackknifes; jackknifed; jackknifing
: to bend in the middle like a folding jackknife
especially, of a large truck : to have the back part slide out of control toward the front part
The truck jackknifed on the icy road.

Cite this Entry

“Jackknife.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/jackknife. Accessed 23 Jun. 2026.

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