noun:

1. A machine used in ropewalks to stretch the yarn.
2. A claw for anchoring a portable machine.
3. A bad-tempered person.
4. An unsold book that is returned to the publisher.
5. A cudgel made of the wood of the crab tree; a crabstick.
6. Short for carabiner.
7. The meat of this crustacean, served as food; crabmeat.
8. A form of windlass, or geared capstan, for hauling ships into dock, etc.
9. A playing card with the rank of three.
10. The angle by which an aircraft's nose is pointed upwind of its groundtrack to compensate for crosswinds during an approach to landing.
11. A movable winch or windlass with powerful gearing, used with derricks, etc.
12. The tree species Carapa guianensis, native to South America.
13. The crab apple or wild apple.
14. The tree bearing crab apples, which has a dogbane-like bitter bark with medical use.
15. An infestation of pubic lice (Pthirus pubis).
16. A crustacean of the infraorder Brachyura, having five pairs of legs, the foremost of which are in the form of claws, and a carapace.
17. A position in rowing where the oar is pushed under the rigger by the force of the water.
18. A defect in an outwardly normal object that may render it inconvenient and troublesome to use.

verb:

1. To drift or move sideways or to leeward (by analogy with the movement of a crab).
2. To move (a camera) sideways.
3. To fly slightly off the straight-line course towards an enemy aircraft, as the machine guns on early aircraft did not allow firing through the propeller disk.
4. To fish for crabs.
5. To be ill-tempered; to complain or find fault.
6. To ruin.
7. To navigate (an aircraft, e.g. a glider) sideways against an air current in order to maintain a straight-line course.
8. To back out of something.
9. To irritate, make surly or sour.
10. To cudgel or beat, as with a crabstick.
11. To complain.