American Oxford Thesaurus
dumbfounded
dumbfounded adjective a dumbfounded audience: astonished, astounded, amazed, staggered, surprised, startled, stunned, confounded, nonplussed, stupefied, dazed, dumbstruck, open-mouthed, speechless, thunderstruck; taken aback, disconcerted; informal flabbergasted, flummoxed, bowled over, blown away, floored.
Oxford Thesaurus
dumbfounded
dumbfounded adjective when you told me I had won I was dumbfounded: astonished, astounded, amazed, staggered, surprised, startled, stunned, confounded, nonplussed, stupefied, dazed, dumbstruck, open-mouthed, agape, speechless, at a loss for words, thunderstruck, goggle-eyed, wide-eyed; taken aback, thrown, shaken, unnerved, disconcerted, discomposed, bewildered; informal flabbergasted, floored, flummoxed, knocked for six, knocked sideways, knocked out, bowled over, blown away, unable to believe one's eyes /ears; Brit. informal gobsmacked.
Sanseido Wisdom Dictionary
dumbfounded
d ù mb f ó und ed ⦅まれ ⦆d ù m -/-ɪd /形容詞 〈人が 〉 «…に /…を見て /…して /…ということに » (物が言えないほど )驚いて [唖然 (あぜん )として ]いる «by /at /to do /that 節 » ; 〈表情 人などが 〉唖然とした .