have someone by the balls
Have someone at one's mercy, as in You have to pay up—they've got you by the balls. The balls here allude to the male genitals. [Vulgar slang; early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.