late 12c., mēk, "gentle or mild of temper; forbearing under injury or annoyance; humble, unassuming;" of a woman, "modest," from a Scandinavian source such as Old Norse mjukr "soft, pliant, gentle," from Proto-Germanic *meukaz (source also of Gothic muka-modei "humility," Dutch muik "soft"), a word of uncertain origin, perhaps from PIE *meug- "slippery, slimy." In the Bible, it translates Latin mansuetus from Vulgate (for which see mansuetude). Sense of "submissive, obedient, docile" is from c. 1300.
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. Compare ancient Greek aitnaios "an unknown fish celebrated for its marital constancy;" alphēstēs, the wrasse, "a fish with a singular and unsavoury reputation ... a byword for the incontinent and lewd" (both in Thompson, who also notes that the hermaphroditic nature of certain fishes, discovered by modern naturalists, was known to Aristotle). The exclamation holy mackerel is attested from 1876.