get something on someone

Also, have something on someone. Obtain or possess damaging knowledge about someone. For example, They hoped to get something on the candidate, or Once Tom had something on his boss, he knew he would not be pressured again. [c. 1920] Also see be on to.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.