Where does “saltpetre” come from?

saltpetre (Middle English) comes from Old French salpetre, from Medieval Latin salpetra, from Latin sāl petrae.

Ancestry of “saltpetre”, step by step

Step	Language	Word	Meaning
1	Old French	salpetre	—
2	Medieval Latin	salpetra	—
3	Latin	sāl petrae	—

saltpetre — Etymology, Origin & Word History | Etymology Explorer