According to Frisk of onomatopoeic origin, from μῡ (mū). According to Beekes there are no plausible comparanda and the word is Pre-Greek. Others compare it to Common Slavic *myslь (“idea, thought”), as if via Proto-Indo-European *muHdʰ-.
μῦθος • (mûthos) m (genitive μύθου); second declension
- something said: word, speech, conversation
- public speech
- (mostly in plural) talk, conversation
- advice, counsel, command, order, promise
- the subject of a speech or talk
- a resolve, purpose, design, plan
- saying, proverb
- the talk of men, rumor, report, message
- tale, story, narrative,
- tale, legend, myth
- (in Attic prose) a legend of the early Greek times, before the dawn of history
- a professed work of fiction, fable, such as those of Aesop
- the plot of a tragedy
The scholia on the Odyssey 21.71 says that μῦθος (mûthos) is Aeolic for μόθος (móthos), but compare μυθιήτης (muthiḗtēs). It is used there in the significance of "resolve, purpose."