As a noun crab
is a crustacean of the infraorder brachyura, having five pairs of legs, the foremost of which are in the form of claws, and a carapace or
crab can be the crab apple or wild apple or
crab can be the tree species , native of south america or
crab can be short for carabiner.
As a verb crab
is to fish for crabs or
crab can be (obsolete) to irritate, make surly or sour.
As an adjective ridiculous is
deserving of ridicule; foolish; absurd.