[have] {sb} by the short hairs

to have complete control over another individual

The exact origin of the idiot "have someone by the short hairs" is uncertain. However, it is believed to have its roots in the idea of physical control or manipulation. The phrase metaphorically refers to having a person grasped or controlled by a sensitive and vulnerable area, such as the short hairs at the back of the neck, which are difficult to escape from without resistance. It is often used when discussing power dynamics or relationships where one individual holds leverage over someone else.

The detective had the suspect by the short hairs, possessing undeniable evidence of their guilt.
The strict teacher had the misbehaving student by the short hairs, threatening to report their actions to the principal.
The powerful politician had the compromised official by the short hairs, ensuring their loyalty through threats of exposure.
The cunning loan shark had the indebted businessman by the short and curlies, making him do whatever was necessary to repay the debt.
The strict coach had the underperforming athlete by the short and curlies, threatening to kick them off the team if they didn't improve.