noun pl. jackknives -·knives′ (-nīvz′)

1. a large pocketknife
2. a dive in which the diver keeps his knees unbent, touches his feet with his hands, and then straightens out just before plunging into the water

Etymology: jack- + knife

transitive verb jackknifed -·knifed′, jackknifing -·knif′·ing

1. to cut with a jackknife
2. to cause to jackknife

intransitive verb

1. to bend at the middle as in a jackknife dive
2. to turn on the hitch so as to form a sharp angle with each other: said of a vehicle and its trailer