I. ˈjak-ˌnīf noun
Date: 1711
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
II. verb
Date: 1806
transitive verb
1. : to cut with a jackknife
2. : to cause to double up like a jackknife
intransitive verb
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