A To blot one's copybook means to commit some gaffe that spoils one's record. It's mainly a British or Commonwealth phrase, though rather old-fashioned. ... It wasn't until the era of the copybook was almost over that the phrase took on a figurative meaning.

What does blot his copybook mean?

British, informal. : to harm one's own reputation The star football player blotted his copybook by using bad language in an interview.

What does copybook mean?

: a book formerly used in teaching penmanship and containing models for imitation.