blot one's copy book

• blot one's copybook

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blot one's copy book (third-person singular simple present blots one's copy book, present participle blotting one's copy book, simple past and past participle blotted one's copy book)

1. (idiomatic) To damage one's own reputation through bad behavior.
   • 1935, Dorothy L. Sayers, Gaudy Night:

     Now, it was the College that had blotted its copybook

   • 2023, Theodore Dalrymple, “From Duty to Decadence”, in City Journal‎^[1]:

     Indeed, it was not uncommonly heard that the crown should skip a generation and be conferred on Prince William, who, so far at least, had not blotted his copybook and was more popular than his father.