crabba

Old English crabba comes from Proto-Germanic *krabbō, likely from Proto-Indo-European *gerbh- meaning 'to scratch, to carve.' The word describes the animal's most distinctive action: clawing, scratching, gripping with its pincers. German Krabbe, Dutch krab, and Old Norse krabbi all share the root. The crab was named for what it does with its hands.

The crab gave English several words by metaphorical extension. 'Crabby' (irritable, snappish) appeared in the 1500s — the temperament of something that pinches. 'Crabbed' (illegible, cramped handwriting) came earlier, in the 1200s — letters scratched sideways like a crab's walk. The crab apple, sour and small, may also derive from the same root, though this is debated.

Aristotle devoted considerable attention to crabs in Historia Animalium. He correctly noted their regeneration of lost limbs, their molting cycle, and the difference between true crabs and hermit crabs. The constellation Cancer is a crab — named by Babylonian astronomers and adopted by the Greeks. The Tropic of Cancer takes its name from the constellation. The crab's sideways walk circles the globe.

Horseshoe crabs (Limulus polyphemus) are not crabs at all — they are more closely related to spiders. Their blue blood contains a chemical (limulus amebocyte lysate, LAL) that detects bacterial contamination, making it one of the most medically important substances on earth. Every injectable drug and every implant device is tested with horseshoe crab blood. The not-crab saves lives.