ax to grind
A selfish aim or motive, as in The article criticized the new software, but the author had an ax to grind, as its manufacturer had fired his son. 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. "Having an ax to grind" then came into figurative use for having a personal motive for some action. [Mid-1800s]
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