"damp and close, warm and humid," 1746, with -y (2) + obsolete mug "a fog, mist," from Middle English mugen "to drizzle" (of a fog or a mist, late 14c.), from a Scandinavian source such as Old Norse mugga "drizzling mist," which is possibly from PIE *meug- "slimy, slippery" (see mucus).
