1730, hypothetical inflammatory principle, formerly believed to exist in all combustible matter, from Modern Latin (1702), from Greek phlogiston (1610s in this sense), neuter of phlogistos "burnt up, inflammable," from phlogizein "to set on fire, burn," from phlox (genitive phlogos) "flame, blaze" (see bleach, v.). Theory propounded by Stahl (1702), denied by Lavoisier (1775), defended by Priestley but generally abandoned by 1800. Related: Phlogistic; phlogisticated.