Other forms: fogs, fogging
Fog is thick low cloud at ground level that hides everything more than a short distance away. Thick fog, fog on the motorway, the fog lifted by ten.
The meteorological line between fog and mist is visibility: below a kilometre it is fog, above it is mist. That is a real definition and forecasts use it. It is uncountable and the verb is impersonal — it is foggy — while to fog up is what a window does. Figuratively, a fog is a state of confusion, and the fog of war is a fixed phrase meaning the uncertainty of a battle as it happens.
1nounfogThick cloud at ground level that makes it hard to see.
“The fog came in off the sea in about ten minutes.”
2nounstateA state of confusion.
“The first week was a complete fog.”
3verbbecomeTo become covered in condensation.
“The windscreen fogged up straight away.”
Too few real examples of this word to estimate which sense is commonest.
fog is a weather condition.
Also accepted in games: mist
The word came into English from a Scandinavian source and originally meant long grass left standing over winter — the damp, sour stuff at the bottom of a field. The weather sense may have come from the mist that hangs over exactly that kind of ground.
Proto-Indo-European
*pug- — billow, bulge, drift
Proto-Germanic
*feukaną — to whisk, blow
Danish
fog — spray, shower, drift, storm
English, today
fog
Built from
Origin unknown; but probably of gmq origin. Probably either a back-formation from foggy (“covered with tall grass; thick, marshy”), from the earlier-attested fog (“tall grass”) (see below), or from or related to Danish fog (“spray, shower, drift, storm”), 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”).
Etymology from Wiktionary, CC BY-SA