Why do we say 'head over heels'?

Think about the phrase head over heels for a moment. Your head is, under ordinary circumstances, over your heels at all times. You at this moment almost certainly have your head over your heels. The phrase, taken literally, describes the perfectly unremarkable state of standing upright; hardly the sort of image we reach for when describing the earth-tilting sensation of falling in love. The original version of the expression, it turns out, had the phrase in the right order.

For several centuries before head over heels came along, English speakers said heels over head. This physically logcal version of the expression has been in use for many hundreds of years; we have a number of citations from the 17th century for heels over head. In all these cases it simply means somersault.

Note that to Somerset, is to tosse heels over head, and to light on heeles again, the word is frequent amongst tumblers at this day.
— The devill seen at St. Albons, 1648

and thereupon leaning himself, as upon the only supporters of his body, he incontinently turned heels over head in the aire, and streight found himself betwixt the bowe of the saddle in a good settlement. Then with a summer-sault springing into the aire again, he fell to stand with both his feet close together upon the saddle.
— François Rabelais,  Gargantua et Pantagruel, 1653

It is unclear why the words got swapped around. What is clear is that when writers eventually did begin putting the head before the heels, they still meant it in the physical sense. Around the end of the 17th century writers (especially translators) began using head over heels, still as a bodily description, and with no romance in sight.

… the Knight, all Fire and Tow, without any humming and hawing, or considering the cursed Risco he runs, but trusting to his Iron Enclosure, and only recommending himself to God and his ador'd Mistress, fetches a Jump, and darts himself head over heels down through the seething Puddle.
— Don Quixote, 1687

…she should come in for the Second Course to make up his Meal; and thereupon made but one leap down Stairs; yet tumbling Head over Heels at the bottom, beat all the Custards in pieces.
— The French Rogue, 1694

The romantic sense enters later, and somewhat mysteriously. The first known use of head over heels to mean being consumed by love appears in a 1711 translation of the ancient Greek writer Lucian, where a lovesick figure is described as "reeling and tumbling Head over Heels" in a state of deep, distracted thought. For much of the 18th century the “tumbling” sense and the “fallen in love” sense appear to have co-existed.

The older form, heels over head, has never entirely disappeared. It still surfaces occasionally, usually in contexts that want to emphasize the literal, physical tumbling, or simply the general chaos of being turned upside down. But head over heels, illogical as it is, has claimed the romantic territory as its own. Perhaps the inversion is the point. Love, after all, is not known for putting things in their proper order.