head over heels
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 phrasal elegance.
The Kindle version of Anthony Trollope's works (19th century): "Complete Works of Anthony Trollope (Illustrated)" has 30 instances of characters being either "...over head and ears in debt" or "...over head and ears in love".
Maybe the phrase over head and ears was part of the transition of "heels over head" to "head over heels".
- Tumbling upside down.
- She tripped and rolled head over heels down the hill.
- At top speed; frantically.
- Hearing the noise in the dark, the children ran head over heels back home.
- Hopelessly smitten.
- He was head over heels in love with the girl next door.
- (tumbling): arse over tit, ass over teakettle, base over apex
- (frantically): full tilt, full throttle, like mad