Enormous or gigantic; of an exceedingly large size or scale.
Derived from 'Brobdingnag,' the land of giants in Swift's 'Gulliver's Travels,' with the addition of '-ian' (a suffix forming adjectives). The word entered English as a common adjective meaning 'enormous' based on the fictional reference.
It's remarkable that a made-up place name from a 300-year-old novel became a real English adjective! 'Brobdingnagian' is one of the clearest examples of how fiction can permanently change a language—we literally borrowed Swift's fictional world to describe real-world enormity.
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