be head over heels (in love)

To be completely enamored of someone, typically a new romantic partner. Oh, I know he's head over heels in love with Christina—he won't stop gushing about her! We used to be head over heels, but now we just annoy each other most of the time.

head over heels (in love), to be/fall

So completely that one is upside down. This expression began life as heels over head, a far more logical description of being turned upside down, and appeared in print in a collection of Early English Alliterative Poems dating from ca. 1350. Four hundred years later an unknown poet turned the saying around: “He gave [him] such an involuntary kick in the face as drove him head over heels” (The Contemplative Man, 1771). This corruption stuck, but the principal sense in which the term is now used dates only from the nineteenth century. An early appearance in print is in David Crockett’s Narrative of the Life of David Crockett (1834): “I soon found myself head over heels in love with this girl.”