Origin: 1275–1325; earlier salt peter; r. ME sal peter, salpetre < ML salpetrē, for L sal petrae salt of rock, so called because it commonly encrusts stones
"potassium nitrate," 1501, earlier salpetre (c.1325), from O.Fr. salpetre, from M.L. sal petrae "salt of rock," from L. sal "salt" (see salt) + petra "rock, stone." So called because it looks like salt encrusted on rock.
[sawlt-pee-ter] 
