fishmonger (n.)

"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.

Old English mangere "merchant, trader, broker," agent noun from mangian "to traffic, trade," from Proto-Germanic *mangojan (source also of Old Saxon mangon, Old Norse mangari "monger, higgler"), from Latin mango (genitive mangonis) "dealer, trader, slave-dealer," which is related to mangonium "displaying of wares."

Not in Watkins or de Vaan, but Buck (with Tucker) describes it as "one who adorns his wares to give them an appearance of greater value" and writes it is probably a loan-word based on Greek manganon "means of charming or bewitching."

Used in combinations in English at least since 12c. (fishmonger, cheesemonger, etc.); since 16c. chiefly with overtones of petty and disreputable (for example ballad-monger "inferior poet," 1590s; scandal-monger).

It is a curious instance of the degradation through which words go, that what was in the Saxon period the designation for the most elevated description of merchant, mangere, is now only a term for small dealers, and principally in petty wares, monger. [Thomas Wright, "Anglo-Saxon and Old English Vocabularies," 1884]