silverfish (n.)

Old English seolfor, Mercian sylfur "silver; money," from Proto-Germanic *silabur- (source also of Old Saxon silvbar, Old Frisian selover, Old Norse silfr, Middle Dutch silver, Dutch zilver, Old High German silabar, German silber "silver; money," Gothic silubr "silver"), which is of uncertain origin.

It seems to be Germanic/Balto-Slavic (source also of Old Church Slavonic s(u)rebo, Russian serebro, Polish srebro, Lithuanian sidabras "silver"), but has long been presumed to be a Wanderwort (a loan-word that has spread among several languages) displacing the usual IE word for the metal (represented by Latin argentum; see argent).

Basque zilharr "silver" usually is considered a loan-word from West Germanic, but the Germanic form lately has been compared to old Celtic words used in Spain, and because the rest of Celtic uses the argentum word, this suggests the borrowing might be in the other direction, and Germanic word might be from "a Hispano-Celtic innovation due to an Iberian donor language. In this connection, the old comparison of Basque zilharr is attractive" [Boutkan].

As an adjective from late Old English (also silvern). As a color name from late 15c. Of voices, words, etc., from 1520s in reference to the metal's pleasing resonance; silver-tongued is from 1590s. The silver age (1560s) was a phrase used by Greek and Roman poets. Chemical abbreviation Ag is from Latin argentum "silver."

"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.