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 Hērakles, 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.
1822 in the figurative sense, "violently making conformable to standard, producing uniformity by deforming force or mutilation," from Procrustes, name of the mythical robber of Attica who seized travelers, tied them to his bed, and either stretched their limbs or lopped of their legs to make them fit it. With ending as in Herculean. By 1776 as Procrustian. The figurative image, though not the exact word, was in English at least from 1580s.
The name is Greek Prokroustēs "one who stretches," from prokrouein "to beat out, stretch out," from pro "before" (see pro-) + krouein "to strike," from PIE *krou(s)- "to push, bump, strike, break" (source also of Russian krušit' "to strike, stamp," Lithuanian kraušyti "to stamp off;" Russian kroxa "morsel, crumb;" Lithuanian krušti "to stamp, push (apart)").