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All answers having "Dumbfound" as clue:

3 Character Answers:

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AWE

WOW



4 Character Answers:

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DAZE

STUN



5 Character Answers:

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AMAZE

FLOOR



6 Character Answers:

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BOGGLE



7 Character Answers:

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NONPLUS

PERPLEX

STUPEFY



8 Character Answers:

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ASTONISH