Simple past tense and past participle of jack-knife.(verb)
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Use "jack-knifed" in a sentence
"They pondered this for a minute; then the woman said something in an undertone to the little girl, and both backs jack-knifed as the slaughter recommenced."
"It was a fearsome thing, of a kind to wake the dead, and I blew and blew, but the jack-knifed backs never unclasped."
"Ice-coated roads were nearly empty in Dallas, where the few motorists who braved the unfamiliar terrain slowed to a crawl as they passed jack-knifed tractor-trailers on slick highways."