AXE TO GRIND
Meaning- something to gain for yourself for a selfish reason; flattery or trickery used to get a favour from another person.
Usage- He always flatters me ,but I think he has an axe to grind .
Story- in the early 1800s a man wrote a story in a newspaper about how ,when he was a boy, a man used flattery to trick him into sharpening the man’s axe. The boy turned the heavy grindstone while the man held his axe against it because the man said the boy was a great axe grinder smart and strong. The man didn’t pay the boy or even thank him. Insted he scolded him for wasting time. Since this idiom came into use.