ax to grind

Etymology:

• This idiom is originated back in the early 19th century, when a man wrote his story in a newspaper about one of his childhood’ events. He stated that once a man praised him that you are a strong boy and can sharpen my ax perfectly. He turned the heavy grindstone whereas the man kept his ax in it. After such hard labor, that man didn’t pay and even thank him for the help. Despite, that man blamed him for getting late from school.