Merriam-Webster's Advanced Learner's Dictionary:
1crab /ˈkræb/ noun, pl crabs
1 a [count] : a sea animal that has a hard shell, eight legs, and two large claws - see color picture
- see also hermit crab, king crab
b [noncount] : the meat of a crab eaten as food
• We had crab for dinner.
2 crabs [plural] medical : a medical problem in which very small insects (called lice) live in the hair surrounding a person's sexual organs
3 [count] US informal : an unhappy person who complains a lot : grouch
• I don't know why she's been such a crab lately.