"jackknife" in the noun sense
1. clasp knife, jackknife
a large knife with one or more folding blades
2. jackknife
a dive in which the diver bends to touch the ankles before straightening out
"jackknife" in the verb sense
1. jackknife
dive into the water bending the body at the waist at a right angle, like a jackknife
Source: WordNet® (An amazing lexical database of English)
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