A: John seems very interested in your new project.

B: His interest may be sincere, but I suspect he has an ax to grind.

A: What do you suspect?

B: I’m afraid he is trying to find a loophole in my proposal.

Note: This idiom means to “have a dispute to take up with someone or, to have an ulterior motive or private ends to serve. This frequently used idiom comes from a story by Charles Miner, published in 1811, about a boy who was flattered into turning the grindstone for a man sharpening his ax. He worked hard until the school bell rang, whereupon the man, instead of thanking the boy, began to scold him for being late and told him to hurry to school. The man scolded the child to avoid giving him a dime or at least a “thank you” for his help.