n. 1 a any of numerous ten-footed crustaceans having the first pair of legs modified as pincers. b the flesh of a crab, esp. Cancer pagurus, as food. 2 (the Crab) the zodiacal sign or constellation Cancer. 3 (in full crab-louse) (often in pl.) a parasitic louse, Phthirus pubis, infesting hairy parts of the body and causing extreme irritation. 4 a machine for hoisting heavy weights. catch a crab Rowing effect a faulty stroke in which the oar is jammed under water or misses the water altogether. crab-grass US a creeping grass infesting lawns. crab-pot a wicker trap for crabs. crablike adj.
[ OE crabba, rel. to ON krafla scratch ]