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.