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
David Faris, Newsweek, 2 Feb. 2025
The trailer of the truck turned sideways in jackknife fashion and was blocking all northbound lanes.
—Aaron Valdez, The Enquirer, 20 Dec. 2024
Christopher Cann, USA TODAY, 5 Mar. 2025
One of crashes was captured on camera by Gracie Fudge, who caught the terrifying moment when a semi-truck slid across multiple traffic lanes and jackknifed on a highway in Kansas City Saturday.
—Saman Shafiq, USA TODAY, 6 Jan. 2025
See All Example Sentences for jackknife
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Noun
Mendoza hit the car in front of him, causing him to jackknife, which in turn caused his entire load of heavy steel pipes to land on my husband's truck, crushing him and killing him instantly.
—David Faris, Newsweek, 2 Feb. 2025
The trailer of the truck turned sideways in jackknife fashion and was blocking all northbound lanes.
—Aaron Valdez, The Enquirer, 20 Dec. 2024
Verb
The driver of a semitruck collided with another truck that had previously jackknifed under an overpass, the Des Moines Register reported.
—Christopher Cann, USA TODAY, 5 Mar. 2025
One of crashes was captured on camera by Gracie Fudge, who caught the terrifying moment when a semi-truck slid across multiple traffic lanes and jackknifed on a highway in Kansas City Saturday.
—Saman Shafiq, USA TODAY, 6 Jan. 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 11 Jul. 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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