jackknife

American  

[jak-nahyf] / ˈdʒækˌnaɪf /

noun

plural

jackknives

1. a large pocketknife.

2. Fancy Diving. a dive in which the diver bends in midair to touch the toes, keeping the legs straight, and then straightens out.

---

verb (used without object)

jackknifed, jackknifing

1. to bend or double over like a jackknife.

   The prizefighter jackknifed and fell when he was hit in the stomach.

2. (of a trailer truck) to have the cab and trailer swivel at the linkage until they form a V shape, as the result of an abrupt stop or accident.

3. (in diving) to perform a jackknife.

4. to move rapidly at an abrupt angle.

verb (used with object)

jackknifed, jackknifing

1. to cause to jackknife.

   The blow jackknifed the prizefighter.

2. to cut with a jackknife.

adjective

1. resembling a jackknife, as in its shape, function, or manner of opening and folding.

jackknife British  

/ ˈdʒækˌnaɪf /

noun

1. a knife with the blade pivoted to fold into a recess in the handle

2. a former name for a type of dive in which the diver bends at the waist in midair, with his legs straight and his hands touching his feet, finally straightening out and entering the water headfirst: forward pike dive

---

verb

1. (of an articulated lorry) to go out of control in such a way that the trailer swings round at an angle to the cab

2. to make a jackknife dive

Etymology

Origin of jackknife

1705–15, jack ¹ ( cf. jockteleg) + knife

Explanation

A knife with a blade that folds closed is called a jackknife. Most jackknives are compact enough to fit in a pocket, but we don't suggest your taking one to school. Another name for a jackknife is a pocketknife. Some jackknives have several different blades, all of which fold tidily (and safely) into the body of the knife, while others unfold to reveal a single blade. The bend of a jackknife inspired the name of the dive also called the jackknife, in which the diver's body folds into a V shape and then straightens out before the diver hits the water.

Example Sentences

The keys for these models are jackknife fobs, wherein the metal blade of the key can fold in and out of the plastic fob.

From Washington Times • Mar. 1, 2023

The officer had gotten out of a vehicle after seeing a semi-trailer jackknife on the road and the patrol car was struck by a passenger car.

From Seattle Times • Nov. 10, 2022

So you get into that situation, the dynamics of sliding on ice — you hit the brakes, and trucks tend to jackknife.

From Washington Post • Jan. 8, 2022

The marlin saw me – with that big eye – and did a jackknife.

From The Guardian • Aug. 10, 2020

I reached into my pocket and touched the horn-handled jackknife, then waved again.

From "The Glass Castle" by Jeannette Walls