A crab is a sea creature with a hard shell, eight walking legs, and two large claws called pincers. Crabs walk sideways in a distinctive scuttling motion, which looks quite funny if you've ever watched one hurry across a beach. They use their powerful claws to catch food, defend themselves, and sometimes communicate with other crabs.

Crabs live in oceans all over the world, from shallow tide pools to deep ocean floors. Some species, like hermit crabs, don't have hard shells of their own and instead move into empty seashells for protection. When a hermit crab outgrows its home, it searches for a bigger shell to move into. Many crabs are excellent swimmers, while others prefer to bury themselves in sand or hide under rocks.

People also use crab as a verb meaning to complain in a grumpy, persistent way. If your brother keeps crabbing about having to do the dishes, he's complaining about it instead of just doing the chore. Someone in a crabby mood is irritable and hard to please, perhaps because they didn't sleep well or are having a frustrating day.

The connection between the grouchy meaning and the animal isn't entirely clear, but it might relate to how real crabs can seem cranky and defensive, snapping their claws at anything that comes too close.