Take the side of, support, defend. For example, Dad will always go to bat for his kids. This term originated in baseball, where it means simply substituting for another batter, but it is the idea of helping one's team in this way that has been transferred to more general use. [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.