jackknife clamEnglishOxford New American Dictionaryjackknife clam /ˈjakˌnīf klam, ˈdʒækˌnaɪf klæm/ ▸ noun North American a burrowing bivalve mollusk with a long, slender shell that resembles the handle of a straight razor. British term razor shell; also called razor clam. Family Solenidae: Ensis and other genera.