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to blot one's copybook
01
to make a mistake that brings about a negative consequence
What is the origin of the idiom "blot one's copybook" and when to use it?
The idiom "blot one's copybook" originated in the 19th century and refers to making a mistake or committing an error that damages one's reputation or standing. The phrase comes from the practice of schoolchildren using copybooks to practice their penmanship. If a student made a mistake and blotted the page with ink, it would ruin the copybook and the teacher would take note of the error. Today, the idiom is used to describe any action that harms one's reputation or standing, particularly in a professional or social context.
Example
He is blotting his copybook by constantly missing his deadlines.
I fear I have already blotted my copybook by failing my last exam.
She always arrives on time, but she blots her copybook by being rude to her colleagues.
He blotted his copybook by getting caught cheating on the exam.
If you don't improve your behavior, you will blot your copybook and damage your chances for promotion.