Avoir le coup de foudre (pour...)
Foudre means lightning, and un coup de foudre is a lightning bolt - useful vocabulary if you're having a conversation about, well, stormy weather. However if you say j'ai eu LE coup de foudre pour Martin(e,) you're certainly not talking about the weather; you are saying that you fell head-over-heels in love with the said Martin ou Martine. In short: it was love at first sight.
The expression is significantly more dramatic than simply tomber amoureux (falling in love, if you're male) ou tomber amoureuse (if you're female,) and describes more the way you fell in love - suddenly and madly - than the process itself, even if the end result is roughly the same: you are in love. Using the definite article le (the) coup de foudre instead of the undefinite un (a) coup de foudre conveys the idea that what you experienced is a "one in a lifetime" event.
By extension you can also have un coup de foudre for a place - such as a city or a country - or an artwork, or pretty much anything that caused such an intense and sudden emotion that you found yourself as if "electrified."
Up until the early 18th century, un coup de foudre had been in use figuratively to describe an unexpected and usually unpleasant event which generated a lot of amazement. As romanticism gained momentum throughout the 18th century, the expression became increasingly used in relation to the matters of the heart. With lightning representing both speed and fire, and fire being a common symbol for passion, the metaphor soon became a favorite for poets and writers depicting the sudden and violent emotion one felt when cupidon's arrow struck. Interestingly, the idiom didn't immediately loose its negative connotation - in truth, the mysteries of romanticism would require feeling chronically tormented - the more, the better - and preferably for love. Nowadays, however, you can safely tell your Valentine tu es mon coup de foudre; no one will see anything but a happy event in it, tinged with a feeling of wonder, perhaps.
Quand je l'ai vu(e) pour la première fois, ça été le coup de foudre.
When I first saw him/her, it was love at first sight.
Lors de mon voyage en Italie, j'ai eu un coup de foudre pour Florence.
When I traveled to Italy, I fell in love with Florence.
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