What is the meaning of Jack-knife?

A compact folding knife.

The front-dive pike, in which the body folds and unfolds.

An abdominal exercise where knees and torso are raised from a lying position.

A semi-trailer truck accident in which the vehicle mimics the closing of a jack-knife.

A game of skill played with a pocket knife in which participants try to throw or flip the knife to stick upright in the ground at specified points or areas.

Alternative spelling of jackknife.

To fold in the middle, as a jackknife does.

To cause a semi-trailer truck to fold like a jackknife in a traffic accident.

Source: wiktionary.org