Pictogram () - a bag or basket, borrowed for phonetic value. Compare ? (OC *to:?, "bundle > east"). Traditionally explained as a pictogram of a bird settling into its nest, which by analogy with the setting of the sun means "west". This etymology has been disputed (Sears), but Sagart (2004) stands by it.
The word is traditionally reconstructed to have a /*s/-initial in Old Chinese, e.g. /*s?:l/ in Zhengzhang (2003). However, recent scholarship has suggested that the Old Chinese initial should instead be reconstructed as /*s-n?/, one of the reasons being ? appears to be the phonetic in ? (n?i), the archaic graphic variant of ? (OC *n?:?). The new reconstruction, /*s-nr/ in Baxter and Sagart (2014), also accounts for how ? can be the phonetic in ? (sh?n, "to smile"). For more information, see Sagart (2004), Baxter and Sagart (2014) and Nohara (2018).
In light of the new construction with the initial consonant cluster /*s-n?/, the possibilities of the etymology of ?, aside from being cognate with ?/? (q?, "to roost; to rest") (B-S OC /*s-nr/), include:
Related to Chepang(nel?-, "to go down; to sink"), which may be the same etymon as the Written Tibetan word above (Schuessler, 2007).
Related to the root ? (n?, "to stop") (OC /*nr?/ for intransitive form, /*nr?-s/ for transitive form in Baxter and Sagart (2014)).
An Austroasiatic nominal n-infix derivative from the root "go down", as in Old Moncnis("ghat") < Old Moncis("to go down") , with Proto-Austroasiatic *tsn- > Proto-Sinitic *sn-. Therefore, this etymon literally means "the place where one goes down to > Mon "ghat" > OC "nest, west". The base form is ? (OC *?sli:ls) via Austroasiatic (Schuessler, 2007).