silverfish n.

"made of silver," late Old English seolfor, from the noun (see silver (n.), also compare silvern). Of voices, words, etc., from 1520s in reference to the metal's pleasing resonance; silver-tongued is from 1590s. Of hair by 1580s.

The silver age (1560s) was a phrase used by Greek and Roman poets. A silver fox is a North American variety of the common red fox with silver-tipped black hair.  A silver spoon in the literal sense is attested from late 15c.; see spoon (n.). The old figurative expression fish with a silver hook is attested from c. 1600.

The Silver Hook, and the Golden Bait, catch all the Fish upon dry Land. [Defoe, "The Union-Proverb," 1708]

"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.