This letter stood for /ʃ/ in Aramaic. 𐢖 was not adopted into the Arabic alphabet because of the following considerations:
Proto-Semitic has three fricatives that became sibilants in descendant languages: ś, š, s. In Arabic ś gave /ʃ/, but š and s merged to /s/. In Old Aramaic all three sibilants were distinguished, and s was represented by 𐢖 / ס / 𐡎 while 𐢝 / ש / 𐡔 represented both ś and š. This orthography persisted in later Aramaic even to the time of the Nabataeans, when the pronunciation of ś merged with that of s/s/.
So when a scribe wanted to write the Arabic sound /ʃ/ from ś, he did so by using 𐢝 / س purposefully in order to stick to a letter form that looks the same in the cognate Aramaic words. But when he wanted to write an Arabic sound /s/ from Proto-Semitic š, he would defy the Aramaic spelling for this Proto-Semitic phoneme were he to represent this Arabic phoneme by the sign 𐢖 / ס / 𐡎, whereas Arabic-Aramaic word pairs with the Proto-Semitic s are – somewhat by coincidence – rare and there was hence only negligible pressure to employ 𐢖 / ܣ / ס / 𐡎, in total. So the impression was that 𐢝 / ש / 𐡔 / س is the most typical sign to write /s/ while the other possible sign fell out of use.
The fifteenth letter in traditional abjad order, which is used in place of numerals for list numbering (abjad numerals). It is preceded by ن (n) and followed by ع (ʕ).
Diem, Werner (1980), “Untersuchungen zur frühen Geschichte der arabischen Orthographie: II. Die Schreibung der Konsonanten”, in Orientalia (in German), volume 49, issue 1, →DOI, pages 77–81
