We've just experienced truly muggy weather. How did the term get applied to very humid weather conditions?
— Bob Turnbull
The American Meteorological Society’s Glossary of Meteorology defines “muggy” as a colloquial term for warm and especially humid weather. Chicago’s recent spell of days in the 80s and dew points in the middle 70s certainly qualifies.
The origin of “muggy” seems to date back to the 1700s and the Norse word “mugga,” which translates to drizzle or mist — weather conditions which are associated with extremely humid weather. Modern usage often applies the term to the overnight hours to describe a very warm and humid night with little wind that typically follows a hot and humid day.