JACKKNIFE

I. noun Date: 1711 a large strong pocketknife, 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 to cut with a ~, to cause to double up like a ~, intransitive verb to double up like a ~ , to turn and form an angle of 90 degrees or less with each other