"potassium nitrate," a chief constituent of gunpowder, c. 1500, earlier salpetre (early 14c.), from Old French salpetre, from Medieval Latin sal petrae "salt of rock," from Latin sal "salt" (from PIE root *sal- "salt") + petra "rock, stone" (see petrous). So called because it looks like salt encrusted on rock and has a saline taste. Spelling was conformed to salt. Related: Saltpetrous.
updated on December 04, 2021