pilot-fish n.

1510s, "one who steers a ship," especially one who has charge of the helm when the ship is passing in or out of harbor, from French pillote (16c.), from Italian piloto, supposed to be an alteration of Old Italian pedoto, which usually is said to be from Medieval Greek *pedotes "rudder, helmsman," from Greek pedon "steering oar," related to pous (genitive podos) "foot," from PIE root *ped- "foot." The change of -d- to -l- in Latin-derived languages ("Sabine -l-") parallels that in odor/olfactory; see lachrymose.

The transferred or figurative sense "a guide, a director of the course of others" is by 1590s. The literal sense was extended by 1848 to "one who controls a balloon," and by 1907 to "one who flies an airplane."

As an adjective, 1788 as "pertaining to a pilot;" from 1928 as "serving as a prototype," thus the noun pilot meaning "pilot episode" (etc.), attested from 1962. A pilot light (by 1890) is a very small light kept burning beside a large burner to automatically light the main burner when the flow is turned on.

"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.