Lateral stability

The letter 'sin' שֹ is known to have originally been pronounced distinctly from both 'shin' שׁ and 'samekh' ס. The letter 'sin' occurs precisely in words where Arabic has a /ʃ/ sound (like in ship), e.g. עשרה = Ar. /ʕaʃra/, showing that the common ancestor of Hebrew and Arabic had a distinct sound wherever this letter occurs. In fact, some obscure Semitic languages (the Modern South Arabian languages) have preserved this sound.

It's thought that sin was originally pronounced as a voiceless lateral fricative /ɬ/, like Welsh llwyd 'gray'. This sound is pronounced similarly to /l/ (as in land), but without vibration of the vocal cords, as if whispering, and often with a 'hushing' articulation. It's also often found in disordered speech. (I personally know someone who pronounces /ʃ/ as [ɬ] in English.) Some English loanwords show that sin had an l-like sound, like Balsam for Hebrew בשם and Chaldean for כשדי.

/ɬ/ (sin) and /s/ (samekh) must have begun merging quite early, since samekh is often used for sin in later books of the Tanakh. However Samaritans pronounce sin as /ʃ/, merging it with shin rather than samekh.

Why did the letter ש serve to indicate two consonants? The Hebrew alphabet was borrowed from the Phoenicians, for whom the sound /ɬ/ had already been lost, so the letter ש was adopted for this purpose. Actually other letters in the Hebrew alphabet also indicated multiple sounds, but that can wait for a different post...