Where does “じゃ” come from?

じゃ (Japanese) comes from Japanese である, from Japanese で, from Japanese ぜ, from Japanese ぞえ, from Japanese え, from Middle Chinese 衣.

じゃ (Japanese): so, then—used for connecting to a reasonable...

Definitions

Step	Language	Word	Meaning
1	Japanese	である	To be: the copula, with the same function as だ ...
2	Japanese	で	at; with, by, using; indicating the following...
3	Japanese	ぜ	sentence-ending particle signifying emphasis,...
4	Japanese	ぞえ	—
5	Japanese	え	Alternative form of い
6	Middle Chinese	衣	—