Reconstruction:Proto-Sino-Tibetan/muːŋ ~ r/s-muːk

• Proto-Sino-Tibetan: *mruk/mrjuɣ (Coblin, 1986)
  • Proto-Tibeto-Burman: *(r-)maw = (r-)məw, *muːŋ ⪤ **r/s-muːk (Matisoff, STEDT); *r-muw = *r-məw ⪤ *r-muuk (LaPolla, 1987); *(r-)məw, *(r-)muw (Weidert, 1987); *(r-)muw (Benedict, 1972)

Here is a merger of the two roots proposed by Matisoff (above), per LaPolla (1987): "Evidence from Dulong supports the contention in STC (n.236, p.77) that *r‑muuk is an archaic doublet of *r‑muw = *r‑məw" (p.11).

Words meaning "dark, covered, obscure, dull" and the like in Sino-Tibetan languages tend to have the phonesthemic initial *m- followed by a back vowel (Schuessler, 2007).

[1] Fog.

*muːŋ ~ r/s-muːk

1. fog, cloud, mist
2. sky, heaven
3. foggy, misty, dark, dull
4. sullen, menacing

• Old Chinese:

  *mogs (ZS) (*kə.mok-s (B-S),, “fog, mist”); *moːŋ (ZS) (“dark, dim”)

  →   Proto-Vietic:  *k-mɔːk ("fog") (whence Pong /kʰmɔːk/)
        Proto-Tai:  *ʰmoːkᴰ (“fog, mist”) (whence Thai หมอก (mɔ̀ɔk), Lao ໝອກ (mǭk))

  霾 (*mˁrə (B-S), *mrɯː (ZS), “haze”)

  瞀 (*mˁ‹r›uk (B-S), *moːgs, *mroːg (ZS), “dim-sighted”)

  蒙 (*moːŋ (ZS), “misty, drizzly; blind; ignorant, stupid; to cover, to cover up”)

  濛 (*moːŋ (ZS), “drizzly, misty”)

  矇 (*moːŋ (ZS), “blind, dark”)

  霢霂 (*mreːg moːg (ZS, “drizzle”)

  (likely) →   Vietnamese:  mù ("dark; blind")

  • Middle Chinese: 霧 (mɨoH), 霾 (mɣɛi), 瞀 (məuH, mɣʌk), 蒙, 濛, 矇 (muŋ), 霢霂 (mɣɛk muk)

• • →   Japanese:  霧 (む, ​mu)
          Korean:  무 (霧, mu)
          Vietnamese:  vụ (霧)

• • →   Vietnamese:  mù ("fog")

    • Modern Mandarin
      • Beijing: 霧 (wù, /u⁵¹/), 霾 (mái, /maɪ̯³⁵/), 瞀 (mào, /mɑʊ̯⁵¹/), 蒙, 濛, 矇 (méng, /mɤŋ³⁵/), 霢霂 (màimù, /maɪ̯⁵¹⁻⁵³ mu⁵¹/)
• Kamarupan
  • Kuki-Chin
    • Central Chin
      • Lushai [Mizo]: mûk (“dull (of colour)”)
• Himalayish
  • Tibeto-Kanauri
    • Bodic
      • Tibetan
        • Written Tibetan: རྨུགས་པ (rmugs pa, “dense fog”), རྨུ་བ (rmu ba, “fog”), རྨུས་པ (rmus pa, “foggy”), མུག་པ (mug pa, “overcast, troubled”), སྨུག་པ (smug pa, “fog, mist”), སྨུག་པོ (smug po, “dark red, purple-brown”), རྨོང་བ (rmong ba, “obscured”)
• Lolo-Burmese-Naxi
  • Lolo-Burmese
    • Burmish
      • Written Burmese: မိုး (mui:, “sky, rain”), မြူ (mru, “minute particle; mist, haze, fog”), မှိုင် (hmuing, “downcast, mope”), မှိုင်း (hmuing:, “dull (of colour)”), မိုက် (muik, “dark; stupid, foolish”), မောင်း (maung:, “dark red”), မှောင် (hmaung, “dark”) (whence အမှောင် (a.hmaung, “darkness, gloom”))
    • Loloish
      • Northern Loloish
        • Yi (Liangshan): *ꃅ (* mu) (mu) ("sky") (whence ꃅꃴ (mu vut, “sky”), ꃅꈯ (mu ggur, “overhead, in the sky”), ꃅꊫꌦ (mu zyp sy, “dark or weak sun”), ꃅꉒ (mu hxuot, “fog, mist”))
      • Central Loloish
        • Lisu (Northern): ꓟꓴꓻꓗꓴꓸ (mu kú) ~ ꓟꓶꓻꓗꓴꓸ (mɯ kú, “cloud, mist”)

• *s-mun (“dark”)
• *s-maŋ ~ s-mak (“black, ink, deep”)
• *(s/r)-ma(ŋ/k) (“dream”)
• *ma (“no, not, none”)
• Proto-Mon-Khmer: *mhuəl ~ mhəl (“cloud”) (whence Vietnamese mây)
• Proto-Tai: *ʰmɯəjᴬ (“hoarfrost”) - some reflexes of this ST etymon also mean "frost, dew" (e.g. Yangjiang dialect of Yue Chinese); a confusion of coldness-related natural phenomena in tropical languages similarly occurred in Vietnamese sương.
• Proto-Indo-European: *h₃migʰleh₂ (“mist”), *h₃moygʰos (“fog, cloud”) < *h₃meyǵʰ-, *meyǵʰ- (“to flicker, blink, be dark; cloud, mist”) (whence English mist) (→ Thai เมฆ (még))