PS

*ˀišd - backside, lower leg, base, foundation (Kogan 2015: 204)

PWS *šit- ‘buttocks’ may be related, see Kogan 2011: 218

Akkadian
išdu, ištu, ildu, irdu, ešdu, ušdu - damp course, base, foundation (of a building, reign), bottom, root, lower extremities (CAD I 235, AHw. 393)
May be related also to PWS *šit- ‘buttocks’, see Kogan 2011: 218 Note that the word is used in particular in the dual išdān which is found in Akk. usually with nouns denoting body parts or their semantic derivatives.
Ugaritic
ˀišd - leg (DUL 116, Watson 2007: 79)
cf. Akk. ˀišdu (Kogan 2015: 332)
it is debatable whether the word is related to PWS *šit- ‘buttocks’, see Kogan 2011: 218