JACKKNIFE is validvalid

a large pocketknife [n JACKKNIVES] / to fold like a pocketknife [v JACKKNIFED, JACKKNIFES, JACKKNIFING]

103,082 of 196,601 in NWL2023·also valid in CSW24

a large clasp knife [n JACKKNIVES] / to fold up like a jackknife [v JACKKNIFED, JACKKNIFING, JACKKNIFES]