Greek hero, son of Zeus and Alcmene, worshipped by the Romans as a god of strength, c. 1200 (originally in reference to the Pillars of Hercules), also Ercules, from Latin Hercles (Etruscan Hercle), from Greek Herakles, literally "Glory of Hera;" from Hera (q.v.) + -kles "fame," a common ending in Greek proper names, related to kleos "rumor, report, news; good report, fame, glory," from PIE *klew-yo-, suffixed form of root *kleu- "to hear."
Used figuratively in reference to strength since late 14c. Vocative form Hercule was a common Roman interjection (especially me Hercule!) "assuredly, certainly." The constellation so called in English by 1670s.