1830, from French mythe (1818) and directly from Modern Latin mythus, from Greek mythos "speech, thought, word, discourse, conversation; story, saga, tale, myth, anything delivered by word of mouth," a word of unknown origin. Beekes finds it "quite possibly Pre-Greek."
Myths are "stories about divine beings, generally arranged in a coherent system; they are revered as true and sacred; they are endorsed by rulers and priests; and closely linked to religion. Once this link is broken, and the actors in the story are not regarded as gods but as human heroes, giants or fairies, it is no longer a myth but a folktale. Where the central actor is divine but the story is trivial ... the result is religious legend, not myth." [J. Simpson & S. Roud, "Dictionary of English Folklore," Oxford, 2000, p.254]
General sense of "untrue story, rumor, imaginary or fictitious object or individual" is from 1840.