A 90 degree day with 90 percent humidity is an example of a muggy day.
Origin of muggy; from dialect, dialectal mug, mist, drizzle, probably ; from or akin to Old Norse mugga ; from Indo-European base an unverified form meuk- from source muck
Origin of muggyProbably from Middle English mugen, to drizzle; akin to Old Norse mugga, a drizzle.
(comparative muggier, superlative muggiest)
From Old Norse mugga (“drizzle, mist").