1
: a large strong pocketknife
2
: a dive executed headfirst in which the diver bends from the waist and touches the ankles while holding the knees unbent and then straightens out
jackknifed; jackknifing; jackknifes
1
: to double up like a jackknife
… jackknifed into a seat beside me, folding up his six feet eight …—
Lady Bird Johnson
Lady Bird Johnson2
: to turn and form an angle of 90 degrees or less with each other
—used especially of a tractor-trailer combination
Examples of jackknife in a Sentence
Verb
The truck jackknifed on the icy road.
Recent Examples on the Web
New Atlas, 9 Nov. 2025
Artifacts found on the island include a woman’s mirror compact similar to one Earhart was known to carry, a cosmetic jar, a jackknife, and a wooden box that appeared to once contain a sextant, a navigation instrument.
—Katie Hunt, CNN Money, 28 Oct. 2025
Nick Lentz, CBS News, 30 Nov. 2025
The trailer then jackknifed and was hit by a minivan, killing all three people inside the minivan, officials said.
—Adam Sabes, FOXNews.com, 4 Nov. 2025
See All Example Sentences for jackknife
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Noun
No wonder so many outdoorsmen threw up their hands and opted for a jackknife.
—New Atlas, 9 Nov. 2025
Artifacts found on the island include a woman’s mirror compact similar to one Earhart was known to carry, a cosmetic jar, a jackknife, and a wooden box that appeared to once contain a sextant, a navigation instrument.
—Katie Hunt, CNN Money, 28 Oct. 2025
Verb
The Michigan Department of Transportation reported several highway lane closures on its Mi Drive map during the morning hours due to crashes and at least one jackknifed semitruck.
—Nick Lentz, CBS News, 30 Nov. 2025
The trailer then jackknifed and was hit by a minivan, killing all three people inside the minivan, officials said.
—Adam Sabes, FOXNews.com, 4 Nov. 2025
See All Example Sentences for jackknife
Cite this Entry
“Jackknife.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/jackknife. Accessed 5 Dec. 2025.
Copy CitationKids Definition
jackknife
1 of 2 noun
jack·knife
ˈjak-ˌnīf 
1
: a large strong pocketknife
2
: a dive in which the diver bends from the waist and touches the ankles before straightening out
jackknife
2 of 2 verb
: to double up like a jackknife
the trailer truck jackknifed
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