The sibilant is uncertain.
Both Akk. and Gez. forms are likely related to the noun *šVm- 'name'.
The reconstruction is not entirely certain. Akk. iš is only attested in a specific group of texts (the archaic liver models from Mari); moreover, it is sometimes considered to be an equivalent of the terminative marker -iš. Throughout EthS this preposition can only be distinguished as a part of other prepositions, for which one may also suppose other ways of division. See Arakelova 2001:29–30.
PWS *šit- 'buttocks' may be related, see Kogan 2011: 218