Also, let someone have his or her head. Allow someone to proceed as he or she wishes, give someone freedom. For example, He usually gave his assistant his head when it came to scheduling appointments, or Sometimes it's wise for parents to let a teenager have his head. This expression alludes to loosening a horse's reins and letting it go where it wants to. [Second half of 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.