nitre (n.) Look up nitre at Dictionary.com
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.