Old English mist "dimness (of eyesight), mist" (earliest in compounds, such as misthleoðu "misty cliffs," wælmist "mist of death"), from Proto-Germanic *mikhstaz (source also of Middle Low German mist, Dutch mist, Icelandic mistur, Norwegian and Swedish mist), perhaps from PIE *meigh- "to urinate." Greek omikhle "fog;" Old Church Slavonic migla"fog;" Sanskrit mih- "fog, mist," megha "cloud" sometimes are said to be cognates in this secondary sense, but Beekes finds these rather more likely to be from a separate IE root meaning "fog."
Sometimes distinguished from fog, either as being less opaque or as consisting of drops large enough to have a perceptible downward motion. [OED]
Also in Old English in sense of "dimness of the eyes, either by illness or tears," and in figurative sense of "things that obscure mental vision."