√S- root. “privative”
Roots with an s-prefix were common in Elvish throughout Tolkien’s life. In notes from around 1967 discussing the root √SKAL, Tolkien considered giving the s-prefix a more specific sense:
Keep this part so far as it affects SKAL [“cover, veil, cloak, conceal”]. SKAL & KHAL were both probably primitively related, being differentiated from a √KAL, made to distinguish this from KAL which became applied to “light”. s- was primarily “privative” [in] meaning. Cf. √STIN-, grey, compared with √TIN-, sparkling; √SKOR-, rough, marred, unequal or unsymmetrical in shape, to KOR, round (PE17/184).
As interesting as this is, Tolkien mentioned this function nowhere else, and there are plenty of s-prefixed roots that have nothing to do with privation. Furthermore, while √SKAL is mentioned several times in Tolkien’s writings, √SKOR “unsymmetrical” appears nowhere else, and the usual root for “grey” is √THIN, not √STIN.
References ✧ PE17/183-184