1650s, of diseases, etc., from Latin eruptus, past participle of erumpere "to break out, burst," from assimilated form of ex "out" (see ex-) + rumpere "to break, rupture" (see rupture (n.)). Of volcanoes, from 1770 (the Latin word was used in reference to Mount Etna). Related: Erupted; erupting.
word-forming element making adjectives from verbs, meaning "pertaining to, tending to; doing, serving to do," in some cases from Old French -if, but usually directly from Latin adjectival suffix -ivus (source also of Italian and Spanish -ivo). In some words borrowed from French at an early date it has been reduced to -y (as in hasty, tardy).