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
2
: to turn and form an angle of 90 degrees or less with each other
—used especially of a tractor-trailer combination
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Merriam-Webster unabridged
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