fog:  The word fog is something of a mystery. It first appears in the 14th century meaning ‘long grass’, a use which persists in Yorkshire fog, the name of a species of grass. This may be of Scandinavian origin. The relationship, if any, between fog ‘grass’ and fog ‘mist’ is not immediately clear, but it has been speculated that the adjective foggy, which to begin with referred to places overgrown with long grass, and then passed via ‘of grassy wetlands’ to ‘boggy, marshy’ may have given rise via this last sense to a noun fog denoting the misty exhalations from such marshy ground.
A rather far-fetched semantic chain, perhaps, lacking documentary evidence at crucial points, and perhaps Danish fog ‘spray, shower’ may be closer to the real source.
"thick, obscuring mist," 1540s, a back-formation from foggy (which appeared about the same time) or from a Scandinavian source akin to Danish fog "spray, shower, snowdrift," Old Norse fjuk "drifting snow storm." Compare also Old English fuht, Dutch vocht, German Feucht "damp, moist." Figurative phrase in a fog "at a loss what to do" first recorded c. 1600. Fog-lights is from 1962.
"long grass, second growth of grass after mowing," late 14c., probably of Scandinavian origin; compare Norwegian fogg "long grass in a moist hollow," Icelandic fuki "rotten sea grass." A connection to fog (n.1) via a notion of long grass growing in moist dells of northern Europe is tempting but not proven. Watkins suggests derivation from PIE *pu- (2) "to rot, decay" (see foul (adj.)).