avoir le coup de foudre

to experience love at first sight

This expression describes the sudden and intense feeling of falling in love immediately upon meeting someone.

Quand je l'ai vue pour la première fois, j'ai eu le coup de foudre.

When I saw her for the first time, it was love at first sight.

First appeared in 17th century France. The lightning bolt originally meant a sudden, unpleasant surprise. By the 18th century it had shifted to love, with lightning as a metaphor for instant attraction.