"dumbfounding" in the verb sense
1. perplex, vex, stick, get, puzzle, mystify, baffle, beat, pose, bewilder, flummox, stupefy, nonplus, gravel, amaze, dumbfound
be a mystery or bewildering to
"This beats me!"
"dumbfounding" in the adjective sense
1. astounding, dumbfounding, dumfounding
bewildering or striking dumb with wonder
Source: WordNet® (An amazing lexical database of English)
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