dumbfounded - definition and synonyms

adjective 

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/dʌmˈfaʊndɪd/
  1. so surprised that you do not know what to do or say

derived word

dumbfound

verb [transitive]
present tense
I/you/we/theydumbfound
he/she/itdumbfounds
present participledumbfounding
past tensedumbfounded
past participledumbfounded