1540s, from Dan. fog "spray, shower, snowdrift," related to O.N. fok "snow flurry." The word meaning "long grass" (c.1300) may be a different word, but the two may connect via a notion of long grass growing in moist dells of northern Europe. Phrase in a fog "at a loss what to do" first recorded c.1600.
A dense layer of cloud lying close to the surface of the ground or water and reducing visibility to less than 1 km (0.62 mi). Fog occurs when the air temperature becomes identical, or nearly identical, to the dew point.
An opaque or semiopaque condensation of a substance floating in a region or forming on a surface.