From Proto-Vietic *raːwʔ. Cognate with Muong Bi rão, Tho riɛw⁴, Proto-Pong *riɛw⁴. This word is only attested in the more innovative subgroups (Viet-Muong, Tho, Pong), while the more conservative subgroups such as Chutic inheriting reflexes of *p-ɗoː (originally "yeast", cf. Khmu pdoʔ ("yeast")) or some other words.
Considering the paucity of cognates, the lack of indication of semantic shift from earlier meaning and that it is a cultural item, this is probably ultimately a loanword, although the origin is not yet clear. However, compare Proto-Tai *ʰlawꟲ (whence Thai เหล้า (lâo)), as well as these Sino-Tibetan words: Chinese 醪 (OC *ruːw, “alcohol with dregs”) (ZS),Tibetan རུ་མ (ru ma, “curdled milk”), and Jingpho ru (“liquor”).[1]
This is not a cognate nor descendant of Chinese 酒 (OC *tsuʔ) (B-S). As indicated by reflexes in other Vietic languages, Vietnamese ‹r-› is inherited and is not the result of spirantization.