An Etymological Dictionary of Astronomy and Astrophysics

1. Any decapod crustacean of the suborder Brachyura, having the eyes on short stalks and a short, broad, more or less flattened body, the abdomen being small and folded under the thorax. → Cancer; → Crab nebula; → Crab pulsar.
   

2. Any of various other crustaceans, as the hermit crab, or other animals, as the horseshoe crab, resembling the true crabs (Dictionary.com).

Etymology (EN): M.E. crabbe; O.E. crabba, from Germanic *krab(b)- (cf. Low Ger. krabben “to scratch, claw”); PIE base *gerbh- “to scratch;” cf. Gk. graphein “to write.”

Etymology (PE): Xarcang “crab,” from Mid.Pers. karcang, cf. Lori qerženg from kar-, qer- + cang, ženg “claw.” The meaning of the first component, xar/qer, is not clear. It may be related to Av. xruta-, xraoždva- “hard,” as in xruždisma- “hard ground” (from xruždi- + zam-), and to the PIE *qarq- “to be hard.” In that case, the Pers. term for crab would literally mean “hard claw.”