Blot one’s copybook

To blot one’s copybook is to make a mistake or spoil one’s record. The expression derives from children making blots or ink spots on their school copybooks (writing books) and thereby spoiling them, which they used to do, literally, from at least the early 19th century. Blot is an old word meaning a stain or spot, according to the OED probably of Scandinavian origin and dates from the late 14th century, but this particular expression in its figurative sense of making a mistake, dates from the early 20th century. See also Copybook.