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All answers having "Nitwits" as clue:

4 Character Answers:

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SAPS



5 Character Answers:

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ASSES

BOOBS

DODOS

DOLTS

JERKS

SIMPS



6 Character Answers:

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IDIOTS

MORONS



8 Character Answers:

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DOOFUSES

DULLARDS

PINHEADS



9 Character Answers:

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IMBECILES



10 Character Answers:

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BIRDBRAINS