Arabic

qaṣāṣ-, quṣāṣ-, qiṣāṣ- - a place on the back of the head where the hair ends; area of the breast where the ribs meet (BK 2 746 )

ḳaṣīṣ- ‘chest, chest bones’ [ibid.]

Also, ḳays-­ ‘penis’ [ibid. 845]; dissimilation of two «emphatics», with a plausible meaning shift ‘joint’ > ‘penis’. 

PS
*ḳ(ʷ)Vyṣ-, *k(ʷ)Vṣṣ- - joint, point of connection between bones (SED I No. 172 )

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).