To a position higher than another's, as in She was furious when her assistant was promoted over her head. Similarly, go over someone's head means “appeal to a higher authority,” as in Since she couldn't help me, I decided to go over her head and talk to her supervisor. [Mid-1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.