Fog
mid 16th century: perhaps a back-formation from foggy.
wiktionary
Origin uncertain; but probably of North Germanic origin. Probably either a back-formation from foggy(“covered with tall grass; thick, marshy”), from the earlier-attested fog(“tall grass”) (see below), [1] [2] or from or related to Danish fog(“spray, shower, drift, storm”), [2] related to Icelandic fok(“spray, any light thing tossed by the wind, snowdrift”), Icelandic fjúka(“to blow, drive”), from Proto-Germanic *feukaną(“to whisk, blow”), from Proto-Indo-European *pug-(“billow, bulge, drift”), from *pew-, *pow-(“to blow, drift, billow”), in which case related to German fauchen(“to hiss, spit, spray”).
From Middle English fogge(“tall grass”), probably from Norwegian fogg(“tall, worthless grass”); compare Scots fog(“moss; lichen”).
etymonline
fog (n.1)
"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.
fog (n.2)
"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.)).
fog (v.)
1590s (transitive), from fog (n.1). Intransitive use from 1849. Related: Fogged; fogging.
