‘Eyes’ are related to ‘mirror’
A discovery, I propose:
Early Chinuk Wawa < shilakom(pe) > ‘mirror’ and always Chinuk Wawa siyáxus(t) ‘eye, eyes’ seem to be related.
Both evidently trace to the (pan-) Chinookan root -kšt ‘to look; to see’. In Chinookan, you should understand, there’s an appreciable amount of free variation between /k/ & /x/.
This explains the occurrence of “K” in one word but “X” in the other; it’s parallel to the evident relationship between /q/ & /x̣/ in another Chinookan-derived CW set of words, łáx̣ ‘to come out’, łáx̣ani ‘outside’ <=> łáq ‘removed, off’.
Siyáxust — a variant that we don’t happen to find at Grand Ronde — has to have been the original form of the word, we infer from the sounds in the Chinookan root.
And its original meaning, as the 2012 G.R. dictionary astutely shows with its etymology info for this term, has to have been ‘eyes‘ — rather than either a singular ‘eye’ or a ‘face’ (because it’s a dual-number noun in Chinookan!).
The somewhat later siyáxus was quite possibly influenced by a common suffix in Southwest Washington Salish, -us ‘eye; face’. Lower Chehalis Salish and to some extent Lower Cowlitz Salish were spoken by the Lower Chinookans alongside their own languages.
The siyáxus variant did not take terribly long to appear. We find the two forms alternating, from some of the earliest documentation. Could they have coexisted in the times before contact with Euro-Americans?
The siyáxust ~ siyáxus alternation could be another of the countless signs I see in Lower Chinookan of long-term intimate contact with Salish.
Another Chinuk Wawa word from the same Chinookan root, by the way, is the less well-known mət’łáxwəs ‘eye-matter’ (what I grew up calling ‘sleep sand’).