nitre (n.)
c.1400, "native sodium carbonate," from Old French nitre (13c.), from Latin nitrum, from Greek nitron, which is possibly of Eastern origin (compare Hebrew nether "carbonate of soda;" Egyptian ntr). Originally a word for native soda, but also associated from Middle Ages with saltpeter (potassium nitrate) for obscure reasons; this became the predominant sense by late 16c.