[jack.knife] n (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
[2]jackknife vt (1806) 1: to cut with a jackknife
2: to cause to double up like a jackknife ~ vi 1: to double up like a jackknife
2: to turn and form an angle of 90 degrees or less with each other--used esp. of a tractor-trailer combination