This root and its variants were the basis for “mist” words for much of Tolkien’s life. It first appeared as ᴱ√HISI with derivatives like ᴱQ. híse “dusk” and ᴱQ. hiswa “dim, fading” (QL/40), and as an element in ᴱQ. Hisilóme which was glossed “Shadowy Twilights” in the earliest Lost Tales (LT1/112). Thus the early root might have meant “✱dusk, dimness, shadow”. The root was probably also an element in the Gnomish equivalent Hithlum from this period (GL/20), perhaps the result of the sound change whereby [[g|[s] became [θ] before [l]]] in Gnomish.
The sense “haze” and “mist” for ᴱQ. híse first appeared in drafts of the Oilima Markirya poem (PE16/62, 75). In The Etymologies of the 1930s Tolkien gave this root as ᴹ√KHITH with variant ᴹ√KHIS and the gloss “mist, fog”; ᴹ√KHIS was listed first but all the actual derivatives were from ᴹ√KHITH (Ety/KHIS). The root appeared again in Notes on Galadriel’s Song from the late 1950s or early 1960s as √KHIΘ “mist” (NGS, PE17/73).
Tolkien’s continued use of Q. Hísilómë and (Northern) S. Hithlum throughout his life testifies to the enduring nature of this root, though it seems to have shifted in sense from 1910s “✱shadow” to 1930s “mist”, and from s to th.
Tolkien introduced the root ᴹ√MITH in The Etymologies of the 1930s as a blending of ᴹ√MIS “✱wet” and ᴹ√KHITH “mist, fog”, with the derivative N. mith “white fog, wet mist” as in N. Mithrim “✱Mist Lake” (Ety/MITH, RINGI; EtyAC/MITH). As a later addition to this entry Tolkien wrote the adjective N. mith “grey”, and that was the more common use of this word in Tolkien’s later writings. In a 1955 letter to David Masson Tolkien specified that “usage suggests that MIÞ- is paler and whiter, a luminous grey” (PE17/72).