Very large in scale, scope, or extent; possessing or characterized by great amplitude or spaciousness.
From amplitude + -ous suffix. The -ous suffix (Latin -osus) means 'full of' or 'characterized by.' This creates an archaic or formal adjective meaning 'full of amplitude.'
This is a delightfully obscure word—most people say 'spacious' or 'vast,' but amplitudinous reminds us that English reserves grand, Latin-derived words for special moments of pomposity.
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