Meaning & use
jack knife
Last updated
Meanings
nouncountable
A large pocketknife whose blade folds into the handle.
Grammar: Plural: jackknives.
Patterns
- fold a jack knife
He carved the stick with a jack knife.
Grandpa always carried a jack knife in his pocket.
The scout packed a jack knife for the camping trip.
verbintransitive
To bend or fold sharply in the middle, like a folding knife; used when a trailer and truck swing toward each other.
Grammar: No object: the truck jackknifed on the ice.
Patterns
- something jackknifes
The truck jackknifed on the icy highway.
The trailer jackknifed when the driver braked hard.
He jackknifed at the waist to touch his toes.
nouncountablesport
A dive in which the body bends at the hips and then straightens before entering the water.
Grammar: Sport term in diving, also called a pike.
Patterns
- a jack knife dive
She scored well with a clean jack knife dive.
He tried a jack knife off the high board.
The diver did a jack knife before the splash.
Word forms
| plural | jackknives |
|---|---|
| third person singular | jackknifes |
| past tense | jackknifed |
| -ing form | jackknifing |
Common combinations
- jackknife on the road
- A lorry jackknifed on the road and blocked traffic.
- pocket jack knife
- The pocket jack knife has a wooden handle.
- jackknifed truck
- A jackknifed truck closed the motorway.
- jackknife dive
- He practiced the jackknife dive every week.
- jackknife at the waist
- She jackknifed at the waist and laughed.
Synonyms and how they differ
- pocketknife
- Pocketknife is the general American word for a folding knife and does not suggest a particular size.
- penknife
- Penknife is the British word for a small folding knife, smaller than most jack knives.
- fold
- Fold is a general verb for bending, whereas jackknife stresses a sharp and sudden angle.
- pike
- Pike is the technical diving name for the same position.
Word family
Commonly confused: jack knife or jackhammer?
jack knife vs jackhammer
A jackhammer is a heavy drill for breaking concrete, not a knife.
Practice
The truck ____ on the icy road, blocking both lanes.
Choose one answer.
Jackknifed means the truck and trailer folded sharply. The other words are unrelated.
Make it your own
Write a sentence using “jack knife”, then check it and compare it with the examples above.