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All answers having "Dumb" as clue:

3 Character Answers:

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DOH

RHO



4 Character Answers:

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DORA

MUTE



5 Character Answers:

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DENSE

INANE



6 Character Answers:

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STUPID



7 Character Answers:

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ASININE

IDIOTIC

MORONIC



8 Character Answers:

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ESTUPIDO