APAD: Head over heels

Head over heels is a common English idiom primarily used to describe being deeply or completely in love with someone. While most often used in romantic contexts, it can also refer to being extremely enthusiastic about something, such as a new car or hobby. 

Even though "head over heels" is the natural upright position for a person, the idiom persists because it evokes the feeling of a somersault or a literal tumble. It suggests that being in love is such a powerful emotion that it "knocks you off your feet," leaving you disoriented and your world feeling "topsy-turvy" or upside down. 

The phrase first appeared in Middle English as "heels over head," which made logical sense to describe being literally upside down, such as during a somersault or a fall. Interestingly, the modern phrase is actually a "corruption" or reversal of a much older expression.  By the 18th century, the phrase began to flip to "head over heels". This shift is thought to have happened because "head over heels" was more euphonious (it sounded better to the human ear) .  While it originally described physical tumbling, it took on its modern figurative meaning of being "consumed by love" in the early 19th century. Famous figures like Davy Crockett are cited as early users of the phrase in this romantic context.