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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.
blot your copybookBRITISH
If you blot your copybook, you damage your reputation by doing something wrong. Their relationship had been so perfect. Until he'd blotted his copybook with Susan, that is. Note: You can also say that there is a blot on your copybook. In fact, just about the only blot on his copybook so far was a missed penalty against Arsenal 10 days ago. Note: In the past, schoolchildren had `copybooks'. These were books of examples of handwriting, with spaces for the children to copy it.
blot your copybooktarnish your good reputation. British
A copybook was an exercise book with examples of handwriting for children to copy as they practised their own writing.