This is a synonym for "gigantic". It originated with "Gulliver's Travels". After journeying to the land of the Lilliputians (a more common word, synonymous with 'tiny'), Gulliver travels to a second land where giants live: the Brobdignagians. And there's a third common English word from this work of literature. After leaving Brobdignag, Gulliver visits a third land of brutish, uncultured humans: the Yahoos.