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blot (one's) copy book

To tarnish, damage, or ruin one's reputation by behaving badly or committing some mistake or social transgression. Refers to a child's copy book, the blotting (staining with ink) of which ruins one's work. Primarily heard in UK. The local councilor blotted his copy book when it came to light that he had accepted bribes to allow unregulated development projects to go ahead. I really blotted my copy book when I spilled my drink on the visiting dignitary last night.
See also: blot, book, copy

blot your copybook

  (British & Australian)
to do something which spoils someone's opinion of you She blotted her copybook by arriving late to a meeting.
See also: blot, copybook