A crab is any of various crustaceans with four pairs of legs, a pair of pincers, a flattish shell, and a short, broad abdomen folded under its thorax. Several similar animals, such as the crab louse, or the hermit crab[?], are also called crabs.
A crab is any of various machines used to hoist heavy objects.
... is not specified in any of the parts of which the compound is formed. A block-head, for example, is someone whose head is said to be as thick as ...