tar someone with the same brush
US /tɑr ˈsʌm.wʌn wɪð ðə seɪm brʌʃ/
UK /tɑr ˈsʌm.wʌn wɪð ðə seɪm brʌʃ/
1.
to unfairly criticize someone by associating them with a group that is disliked or has a bad reputation, even if they do not deserve it
:
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It's unfair to tar all politicians with the same brush; some are genuinely trying to make a difference.
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Just because a few students cheated doesn't mean you should tar the whole class with the same brush.