crayfish n.

popular name for a stalk-eyed, short-tailed, ten-legged crustacean, Old English crabba, from a general Germanic root (compare Dutch krab, Old High German krebiz, German Krabbe, Old Norse krabbi "crab"), related to Low German krabben, Dutch krabelen "to scratch, claw," from PIE root *gerbh- "to scratch, carve" (see carve). French crabe (13c.) is from Germanic, probably Old Norse. 

The zodiac constellation name is attested in English from c. 1000; the Crab Nebula (1840), however, is in Taurus, the result of the supernova of 1054, and is so called for its shape. Crab stick "white fish meat dyed to resemble crab and pressed into a stick shape" is by mid-1950s. To catch a crab "fall or be thrown due to a mistake in rowing" is from 1785. The crab-louse (1540s), commonly found in pubic hair, is so called for its shape and appearance. Short form crab for this is from 1840; related: Crabs.

"a vertebrate which has gills and fins adapting it for living in the water," Old English fisc "fish," from Proto-Germanic *fiskaz (source also of Old Saxon, Old Frisian, Old High German fisc, Old Norse fiskr, Middle Dutch visc, Dutch vis, German Fisch, Gothic fisks), perhaps from PIE root *pisk- "a fish." But Boutkan on phonetic grounds thinks it might be a northwestern Europe substratum word.

Popularly, since Old English, "any animal that lives entirely in the water," hence shellfish, starfish (an early 15c. manuscript has fishes bestiales for "water animals other than fishes"). The plural is fishes, but in a collective sense, or in reference to fish meat as food, the singular fish generally serves for a plural. In reference to the constellation Pisces from late 14c.

Fish (n.) for "person" is from 1750 with a faintly dismissive sense; earlier it was used in reference to a person considered desirable to "catch" (1722). Figurative sense of fish out of water "person in an unfamiliar and awkward situation" attested by 1610s (a fisshe out of the see in the same sense is from mid-15c.). To drink like a fish is from 1744. To have other fish to fry "other objects which invite or require attention" is from 1650s. Fish-eye as a type of lens is from 1961. Fish-and-chips is from 1876; fish-fingers from 1962.

common name of small, long-tailed freshwater crustaceans, 1620s, a variant of crayfish (q.v.) common in the U.S., but not originally an American form. Also in 19c. American English as a verb, "to back out," in reference to the creature's movements.