To have an axe to grind

Origin

This phrase grinds its way into our language thanks to none other than Benjamin Franklin, the famously witty American polymath. He recounted a childhood anecdote in his autobiography that perfectly illustrates the concept. As a boy, a smooth-talking stranger flattered him into helping sharpen an axe on a grindstone. The stranger promised a shiny new edge, but once Franklin had done all the arduous work of turning the heavy stone, the stranger simply took the axe and left without so much as a thank you. Franklin, realizing he'd been tricked into doing the labor for someone else's benefit, forever after used this experience to describe anyone who pretends friendship or interest while secretly pursuing their own self-serving motives—they literally have an "axe to grind."