"pertaining to or involving intense heat deep in the earth's crust," 1796, coined by Irish scientist Richard Kirwin (1733-1812) from comb. form of Pluto (as god of the underworld) + -ic. Especially in reference to early 19c. geological theory (championed by Hutton) that attributed most of the earth's features to action of internal heat, a theory which triumphed over its rival, neptunism, which attributed them to water. Related: Plutonism; Plutonist.