Ever been so in love that the object of your affections is all you can think about? Then you were head over heels for them. The idiom head over heels describes the action of falling down or doing a somersault. (Originally the expression was heels over head, but it, well, flipped during the 1700s.) In other words, if you are head over heels for someone, you are thoroughly in love with them.

Source: Thesaurus