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
Katie Hunt, CNN Money, 28 Oct. 2025
The senior jackknife had a 10-yard rushing TD, then a punt return for a TD to give the Eagles a 14-12 lead.
—Matt Schubert, Denver Post, 31 Aug. 2025
Adam Sabes, FOXNews.com, 23 Oct. 2025
The landmass burbled over a million square miles of lava, injecting thousands of gigatons of carbon dioxide into the air and jackknifing the temperature by 10 degrees Celsius over thousands of years, killing off most animal life on Earth.
—Peter Brannen, Quanta Magazine, 15 Sep. 2025
See All Example Sentences for jackknife
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Noun
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
The senior jackknife had a 10-yard rushing TD, then a punt return for a TD to give the Eagles a 14-12 lead.
—Matt Schubert, Denver Post, 31 Aug. 2025
Verb
The semi-truck jackknifed as a result and hit the Ram Sprinter, which was pushed into a road sign.
—Adam Sabes, FOXNews.com, 23 Oct. 2025
The landmass burbled over a million square miles of lava, injecting thousands of gigatons of carbon dioxide into the air and jackknifing the temperature by 10 degrees Celsius over thousands of years, killing off most animal life on Earth.
—Peter Brannen, Quanta Magazine, 15 Sep. 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 31 Oct. 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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