head over heels - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Completely, thoroughly, as in They fell head over heels in love. This expression originated in the 1300s as heels over head and meant literally being upside down. It took its present form in the 1700s and its present meaning in the 1800s.