Crustaceans with hard shells and clawed legs that walk sideways, or a parasitic infection.
From Old English 'crabba,' from Proto-Germanic 'krabbaz,' possibly imitative of the crabbing motion. The disease meaning comes from the sideways motion of the louse.
Crabs get their name from the Germanic root imitating their sideways scurrying motion—and the parasitic lice are called 'crabs' because they move sideways too, showing how one word can spread to unrelated meanings through similarity.
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