agile, short-legged, bushy-tailed, medium-sized carnivorous mammal in the weasel family, largely nocturnal and found in forests across the colder parts of the northern hemisphere, c. 1300, martrin, "skin or fur of the marten," from Old French martrine "marten fur," noun use of fem. adjective martrin "of or pertaining to the marten," from martre "marten," from Frankish *martar or some other Germanic source, from Proto-Germanic *marthuz (source also of Old Saxon marthrin "of or pertaining to the marten," Old Frisian merth, Middle Dutch maerter, Dutch marter, Old High German mardar, German Marder, Old English mearþ, Old Norse mörðr "marten").
The ultimate etymology is unknown. Some suggest it is from PIE *martu- "bride," on some fancied resemblance. Or it might be a substrate word or a Germanic euphemism for the real name of the animal, which might have been taboo. In Middle English the animal itself typically was called marter, directly from Old French martre, but martrin took over this sense in English after c. 1400. The form marten is from late 16c., perhaps due to association with the masc. proper name Martin.