blot one's copybook

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.