/ḳiṣrum/
The anatomic meaning ‘joint’ follows from the combination gi-zi-rí bù-tim ‘shoulder joint’ [ibid.]
PS
*ki/uṯ̣r- - joint (of the ribs, fingers, etc.)
(SED I No. 155)
Though reconstructed as an anatomic term on the PS level, the present root is likely related to, or rather derived from, Sem. *k(ʷ)ṯ̣r ‘to bind, knot, fix’