In a manner that is extremely surprising or amazing; in a way that shocks.
Adverbial form of flabbergasting, created by adding -ly to the adjective, following standard English word-formation patterns.
This word is practically never used because 'flabbergastingly' is a mouthful, and English speakers almost always just say 'flabbergasted' instead—showing how adverbs often lose popularity when they become too cumbersome to pronounce.
Complete word intelligence in one call. Free tier — 50 lookups/day.