This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. shocked and speechless.

v. Simple past tense and past participle of dumbfound.

adj. same as astounded.

adj. astonished and confounded.

adj. as if struck dumb with astonishment and surprise

"Callum had stepped back, blinking rapidly, his expression dumbfounded."

surprised   

stunned    perplexed    confounded    amazed    speechless    shocked    flabbergasted    astounded    surprised    astonished   

dumbfound   

bemused    unsure    uncomprehending    speechless    unconvinced    slack-jawed    crestfallen    aghast    open-mouthed    petrify