We found one dictionary that defines the word flabbergastedness:

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1. flabbergastedness: Wiktionary

Definitions from Wiktionary (flabbergastedness) ▸ noun:  The state of being flabbergasted. ▸ Words similar to flabbergastedness ▸ Usage examples for flabbergastedness ▸ Idioms related to flabbergastedness ▸ Wikipedia articles (New!) ▸ Popular adjectives describing flabbergastedness ▸ Words that often appear near flabbergastedness ▸ Rhymes of flabbergastedness ▸ Invented words related to flabbergastedness