have (someone) by the short hairs

To completely control, dominate, or hold power over someone, especially in a difficult or awkward situation. A reference to the hairs on the back of one's neck, though it is often (and incorrectly) assumed that the phrase refers to pubic hairs. Can also be used with the verbs "got" or "catch." After scoring a third goal in the last quarter, the home team really had their opponents by the short hairs. He found out about my painkiller addiction and is using it to blackmail me—he's got me by the short hairs now.