Origin and history of giga-

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.

1680s, from French billion (originally byllion in Chuquet's unpublished "Le Triparty en la Science des Nombres," 1484; copied by De la Roche, 1520); see bi- "two" + million.

A million million in Britain and Germany (numeration by groups of sixes), which was the original sense; subsequently altered in French to "a thousand million" (numeration by groups of threes) and picked up in that form in U.S., "due in part to French influence after the Revolutionary War" [David E. Smith, "History of Mathematics," 1925].

France reverted to the original meaning in 1948. British usage is truer to the etymology, but U.S. sense is said to be increasingly common there in technical writing.

In Italian arithmetics from the last quarter of the fifteenth century the words bilione or duilione, trilione, quadrilione or quattrilione, quintilione, cinquilione, or quinquilione, sestione or sestilione, settilione, ottilione, noeilione and decilione occur as common abbreviations of due volte millioni, tre volte millione, etc. In other countries these words came into use much later, although one French writer, Nicolas Chuquet, mentions them as early as 1484, in a book not printed until 1881. The Italians had, besides, another system of numeration, proceeding by powers of a thousand. The French, who like other northern peoples, took most if not all their knowledge of modern or Arabic arithmetic from the Italians, early confounded the two systems of Italian numeration, counting in powers of a thousand, but adopting the names which properly belong to powers of a million. [Century Dictionary]

For a time in Britain gillion (1961), based on giga-, was tried as "a thousand million" to avoid ambiguity. Compare milliard.