冬 *tˤuŋ > towng > dōng ‘winter’ is clearly related to 終 *tuŋ > tsyuwng > zhōng ‘end’. An external cognate is Chepang tyuŋh ‘at end’; ‘be weak, exhausted’. Final *-m in various TB languages may be due to assimilation to a verbal suffix -ma, as in Limbu (Michailovsky) tum-ma (√tums-) ‘to be mature, to be ripe’ (of person, grain or fruit). Forms of this verb with final -m, without the suffix, are Jingpo chatum ‘end, finish’; Proto-Ao a-thəm ‘end, finish’ (Bruhn 14). Along with tyuŋh ‘at end’, Chepang has doublet ending in -m: dyumʔ– ‘at ease after completing work’, dyum ‘terminate, reach end’; ‘at end’. This would be the ‘end’ of the year’s agricultural cyle.
Although OC *-um goes to *-uŋ in western China in early OC, we can be sure that *-uŋ in 冬 is not the western reading of an OC *-um word: if it was, it would end up as *-əm in the easter pronunciation ancestral to MC (Ma and Sagart, forthcoming) and we would have MC *-om or *-im. So *-uŋ > *-um in TB is due to a labial suffix.