The original anatomical meaning is completely lost throughout this group. Semantically similar prepositional use of reflexes of *ṯ̣a(h)r- is admittedly attested in a few other Semitic languages (SED I No. 284). (Kogan 2015:575)
The origin is unknown. It would be tempting to identify it with Gez. ṣaraba ‘to hew,’ ṣarb ‘plank,’ ṣərrāb ‘splinter, shaving’ and its cognates throughout EthS (CDG 563), but this comparison is difficult in view of reliable parallels with ṣ elsewhere in Semitic (notably, in Soq. ṣérob ‘couper,’ LS 357). (Kogan 2015:575)
The origin is uncertain, although an ultimate connection with PES *ṭlf ‘to snatch, to abduct’ is not unlikely. (Kogan 2015:574)