In an extremely large, impressive, or significant way; to a tremendous or enormous degree.
From colossus (a giant statue or something immensely large) + -ally (adverbial suffix). Colossus comes from Greek 'kolossos,' famously referring to the Colossus of Rhodes, one of the Seven Wonders of the Ancient World.
The Colossus of Rhodes was so massive that when it fell, it lay broken for 900 years and people could still see its enormous bronze hands—so 'colossally' literally means as big as something you'd need an ancient civilization to build!
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