an ax to grind

Definition 
A selfish motive. Allegedly this term comes from a cautionary tale by Charles Miner, first published in 1810, about a boy persuaded to turn the grindstone for a man sharpening his ax. The work not only was difficult to do but also made him late for school. Instead of praising the youngster, the man then scolded him for truancy and told him to hurry to school. Other sources attribute it to a similar story recounted by Benjamin Franklin. Whichever its origin, the term was frequently used thereafter and apparently was a cliché by the mid-nineteenth century.