crab: Crab the crustacean [OE] and crab the apple [14] may be two distinct words. The word for the sea creature has several continental relatives (such as German krebs and Dutch krabbe) which show it to have been of Germanic origin, and some of them, such as Old Norse krafla ‘scratch’ and Old High German krapho ‘hook’, suggest that the crab may have received its name on account of its claws.
The origins of crab the fruit are not so clear. Some would claim that it is simply a metaphorical extension of the animal crab, from a perceived connection between the proverbial perversity or cantankerousness of the crustacean (compare crabbed) and the sourness of the apple, but others have proposed a connection with Swedish dialect skrabba ‘wild apple’, noting that a form scrab was current in Scottish English from at least the 16th century. => crayfish[crab etymology, crab origin, 英语词源]