Origin and history of gigantic

c. 1300, "fabulous man-like creature of enormous size," from Old French geant, earlier jaiant "giant, ogre" (12c.), from Vulgar Latin *gagantem (nominative gagas), from Latin gigas "a giant," from Greek Gigas (usually in plural, Gigantes), one of a race of divine but savage and monstrous beings (personifying destructive natural forces), sons of Gaia and Uranus, eventually destroyed by the gods. The word is of unknown origin, probably from a pre-Greek language. Derivation from gegenes "earth-born" is considered untenable.

In þat tyme wer here non hauntes Of no men bot of geauntes. [Wace's Chronicle, c. 1330]

It replaced Old English ent, eoten, also gigant (from Latin). The Greek word was used in Septuagint to refer to men of great size and strength, hence the expanded use in modern languages; in English of very tall and unusually large persons from 1550s; of persons who have any quality in extraordinary degree from 1530s. As a class of stars, from 1912. As an adjective from early 15c. Giant-killer is from 1726.

medical condition causing abnormal increased size, 1854, from Latin gigant- "giant" (see gigantic) + -ism.