He was head over heels in love with the girl next door.
Emerged in the 14th century as "heels over head", which is more literally accurate, as "head over heels" is the more standard state of being. "Heels over head" evolved into "head over heels" in common use departing its literal meaning, probably for reasons of phrasalelegance.
English Wiktionary. Available under CC-BY-SA license.