mackerel (n.)

edible fish of the North Atlantic (Scomber scombrus), c. 1300, from Old French maquerel "mackerel" (Modern French maquereau), of unknown origin; perhaps so called from the dark blotches with which the fish is marked, from Latin macula "spot, stain" (see macula). But the word is apparently identical with Old French maquerel "pimp, procurer, broker, agent, intermediary" (itself attested in English in this sense by early 15c.), a word from a Germanic source (compare Middle Dutch makelaer "broker," from Old Frisian mek "marriage," from maken "to make").

The connection would be obscure, but medieval people had imaginative notions about the erotic habits of beasts. The fish approach the shore in shoals in summertime to spawn. Exclamation holy mackerel is attested from 1876.