Head over heels

Generated image of a couple head over heels in love
Head over heels means to be completely and deeply in love with someone. It can also describe being very excited or enthusiastic about something.

The phrase comes from an older expression, heels over head, which described turning upside down or doing a somersault. Over time the wording changed, and the idea of being flipped upside down became a metaphor for strong emotions.

He fell head over heels for her after their first date.