I. ˈ ̷ ̷ˌ ̷ ̷ noun
Etymology: probably from jack (I) + knife
1. : a large strong clasp knife for the pocket : a large pocketknife
2. : a dive executed headfirst either forward or backward in which the diver beginning usually at the highest point of the dive bends from the waist and touches or clasps his ankles while holding his knees unflexed before straightening out to enter the water
II. transitive verb
1. : to cut with a jackknife
2. : to double up like a jackknife
willing to jackknife himself to get in and out — Springfield (Massachusetts) Republican
3. : to cause to jackknife
the tractor and trailer were jacknifed by the diesel — Springfield (Massachusetts) Union
intransitive verb
1. : to double up like a jackknife
jackknife on the sofa beside the Christmas tree — D.C.Peattie
2. : to turn or rise and form an angle of 90 degrees or less with each other — used especially of a pair of vehicles one of which is attached to and follows the other (as a tractor and its trailer or two railroad cars)
III. adjective
: resembling a jackknife especially in its manner of opening and closing
a jackknife drawbridge
a jackknife door