With k < *ḳ by dissimilation of two «emphatic» consonants.
The form kiṣṣu would exactly correspond to Gez. ḳʷəyṣ, ḳʷəṣ if not for the fact that it can hardly be separated from the Akk forms with m and n, of which kimṣu looks a primary one (cf. also kamāsu, kamāṣu ‘to squat, kneeľ).
Attested in South-Semitic area only. Meanings in Arb., Gez. and Soq. rather diverse; S. Eth. forms may be of a different origin.
Ugr. ḳmṣ also likely belongs to this root (Kogan 2015:336, with fn. 989).