late 14c., "wanting self-restraint," from Old French incontinent (14c.) or directly from Latin incontinentem (nominative incontinens) "immoderate, intemperate, not holding back," from in- "not, opposite of" (see in- (1)) + continens (see continent (adj.)).
Originally chiefly of sexual appetites. General sense of "unable to retain" is from 1640s; medical sense of "unable to control bowels or bladder, unable to restrain natural discharges from the body" is attested by 1828.
He was incontynent, and with fleschely lustes he consumyd alle his tyme. ["Speculum Sacerdotale," 15th century]
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.