Jack knife is a term commonly used to describe a type of diving or a folding knife, but it has several synonyms you might be interested in learning. When referring to diving, the word "Gainer" can be used, which refers to jumping into the water and doing a backflip in the air. When referring to a folding knife, "Penknife" or "Pocketknife" could be an appropriate synonym to use. Another slang term for a folding knife is a "switchblade," which might be the term you're looking for if you're into classic American movies. These are just a few examples of synonyms for jack knife; the English language is vast and diverse, with many words that can convey similar meanings.