ax to grind

Example: “The reviewer clearly has an ax to grind with the director.”

Origin: Often credited to Benjamin Franklin, whose 1791 autobiography tells of being tricked as a boy: a stranger carrying an axe asked young Franklin to demonstrate how a grindstone worked, and Franklin obligingly sharpened the man's axe for him, getting nothing but a lesson in gullibility in return. Franklin, though, draws no figurative conclusion from the story at all — the phrase in its modern sense of a hidden personal motive is actually traced to another Pennsylvania writer, Charles Miner, whose 1810 newspaper piece "Who'll Turn the Grindstone?" appears to be the real first use of "an axe to grind" as an idiom for self-interest dressed up as something else.