mackerel (n.) Look up mackerel at Dictionary.com
edible fish, c.1300, from Old French maquerel "mackerel" (Modern French maquereau), of unknown origin but apparently identical with Old French maquerel "pimp, procurer, broker, agent, intermediary," a word from a Germanic source (compare Middle Dutch makelaer "broker," from Old Frisian mek "marriage," from maken "to make"). The connection is 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.