Source : Webster's Revised Unabridged Dictionary (1913)

Jackknife \Jack"knife`\, n.
   A large, strong clasp knife for the pocket; a pocket knife.

Source : WordNet®

jackknife
     n 1: a large knife with one or more folding blades [syn: {clasp
          knife}]
     2: a dive in which the diver bends to touch the ankles before
        straightening out
     v : dive into the water bending the body at the waist at a right
         angle, like a jackknife
     [also: {jackknives} (pl)]