Today I had to look up whether it would be period to use the phrase "head over heels." Thankfully, it falls within my current writing's time frame!
"Head over heels" derives from "heels over head" in medieval times. At first it was used not to describe romantic feelings, but to express a feeling of disorientation, similar to "topsy turvy" in nature. The "heels over head" described a cartwheel or somersault where you were literally upside down.
The London Annual Register newspaper printed the original phrase in January, 1766: "...Being thrown with great vehemence from a projecting crag, which turned him heels over head." At some point thereafter, the phrase flipped (can you imagine saying "heels over head?" It just doesn't roll of the tongue the same way--and forget what it would have done to the cadence of the Bangles song...)
The first known literary use of the new version comes from Herbert Lawrence's Contemplative Man (1771), which reads: "He gave such a violent involuntary kick in the Face, as drove him Head over Heels."
Leave it to us Yanks to make it all about love. In 1834, our own Davy Crockett used in his Narrative of the life of David Crockett, "I soon found myself head over heels in love with this girl."
Of course, some authors, being the fact-checking sticklers they occasionally are (i.e., when it suits their plot line!) stuck to the original phrase, even as late as the 20th century. L Frank Baum consistently used the older form in his Oz books: “But suddenly he came flying from the nearest mountain and tumbled heels over head beside them.”
And that, my friends, is today's useless trivia!
Great, all I need is one more reason to procrastinate! As if Instagram wasn't enough...