The Collaborative International Dictionary of English v.0.48:

Jackknife \Jack"knife`\, n.
   A large, strong clasp knife for the pocket; a pocket knife.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "jackknife":
   adz, ax, belly buster, belly flop, belly whopper, bread knife,
   bushwhacker, butcher knife, cannonball, carving knife, chaser,
   chisel, cleaver, clipper, clippers, crash dive, dagger, dive, drop,
   fall, gainer, gouge, hack, hatchet, header, hoe, hunting knife,
   knife, lance, lancet, letter-opener, machete, mattock, nippers,
   nose dive, paper cutter, paper knife, parachute jump, paring knife,
   penknife, pick, pickax, pitch, plowshare, plunge, pounce,
   power dive, razor, razor blade, running dive, saw knife, sax,
   scalpel, scissors, scoop, scraper, scuffle hoe, scythe, share,
   shears, sheath knife, sickle, sidecutters, sky dive, snips, spear,
   spokeshave, stationary dive, stoop, surgical knife, swan dive,
   swoop, sword, table knife, wedge