Extremely surprising, shocking, or confusing; capable of leaving someone speechless.
Present participle of 'dumbfound,' used as an adjective to describe something that has the effect of dumbfounding someone.
This is one of those words that actually got more common as 'dumbfounded' (the past tense adjective) became less popular—'dumbfounding' sounds more dramatic and active, like the surprise is still happening.
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