The story
- Old Norse · derived frommugga“drizzle, mist”
- English todaymuggy
From dialectal English mug (“fog, mist; Scotch mist”) + -y, ultimately from Old Norse mugga (“drizzle, mist”); borrowed some time before 1390, when a derived verb, Middle English mugen (“of a fog: to drizzle”), is attested.
Etymology from Wiktionary, CC BY-SA 4.0.