Jackknife (noun)

1. A dive in which the diver bends forward at the waist and brings both legs up and forward to touch the ankles with the knees.
2. A movement where a vehicle's front wheels come up and the back wheels come down causing the back of the vehicle to rise up in a similar way to the dive.

Origin:

It is a term that comes from the late 19th century and is believed to have originated from the name of a specific brand of knife that was used for this type of movement, or from the shape of the blade which resembled a folding jackknife.

Examples:

1. The diver performed a jackknife before entering the water.
2. The driver lost control of the vehicle and it jackknifed on the highway.
3. Jackknife is a dangerous move in diving, as it can cause injury if done incorrectly.
4. A jackknife in trucking refers to a dangerous situation where the tractor and trailer fold in on each other.
5. A jackknife position in gymnastics is when a gymnast brings their legs up and forward to touch their ankles with their knees while bent forward at the waist.