Etymology
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crab (n.1)

popular name for a stalk-eyed, short-tailed, ten-legged crustacean, Old English crabba, from a general Germanic root (compare Dutch krab, Old High German krebiz, German Krabbe, Old Norse krabbi "crab"), related to Low German krabben, Dutch krabelen "to scratch, claw," from PIE root *gerbh- "to scratch, carve" (see carve). French crabe (13c.) is from Germanic, probably Old Norse. 

The zodiac constellation name is attested in English from c. 1000; the Crab Nebula (1840), however, is in Taurus, the result of the supernova of 1054, and is so called for its shape. Crab stick "white fish meat dyed to resemble crab and pressed into a stick shape" is by mid-1950s. To catch a crab "fall or be thrown due to a mistake in rowing" is from 1785. The crab-louse (1540s), commonly found in pubic hair, is so called for its shape and appearance. Short form crab for this is from 1840; related: Crabs.

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