"jackknife" in the noun sense
1. clasp knife, jackknife
a large knife with one or more folding blades
a dive in which the diver bends to touch the ankles before straightening out
"jackknife" in the verb sense
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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