Keep close watch or tight control over, as in Aunt Martha is always riding herd on her bridge club, making sure they follow the rules. This idiom alludes to the cowboy who rides around a herd of cattle to keep them together. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.