n.
Function: transitive verb
Date: 1806
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