Where does “しゃ” come from?

しゃ (Middle Chinese) comes from Old Japanese 汝, from Old Japanese れ — nominalizing suffix used to form pronouns, often from demonstratives: one, thing.

Ancestry of “しゃ”, step by step

Step	Language	Word	Meaning
1	Old Japanese	汝	first-person singular pronoun: I, me;...
2	Old Japanese	れ	nominalizing suffix used to form pronouns, often from demonstratives: one, thing

Every word from Old Japanese れ →Every word from Old Japanese 汝 →