saltpeter (n.)youdaoicibaDictYouDict[saltpeter 词源字典]
"potassium nitrate," c. 1500, earlier salpetre (early 14c.), from Old French salpetre, from Medieval Latin sal petrae "salt of rock," from Latin sal "salt" (see salt (n.)) + petra "rock, stone" (see petrous). So called because it looks like salt encrusted on rock.[saltpeter etymology, saltpeter origin, 英语词源]