Crab1  n. 1 a ten-footed crustacean, with the first pair of legs as pincers. B crab as food. 2 (crab) sign or constellation cancer. 3 (in full crab-louse) (often in pl.) Parasitic louse transmitted sexually to esp. Pubic hair. 4 machine for hoisting heavy weights.  catch a crab rowing jam an oar or miss the water.  crablike adj. [old english]