Avoir un cœur d’artichaut is an almost comical French expression which translates literally to having an artichoke heart. It is used to describe someone who falls in love easily and/or frequently, possibly with several people at the same time — or at least in rapid succession. There is no particular positive or negative connotation to the expression itself; only the context, or the narrator, will tell if the person in question is a sweet, hopeless romantic, or a compulsive, egomaniac heart breaker - although, in all honesty, the liver-friendly vegetable might not be your first choice to describe the latter. As double-standards would have it, the said artichoke's heart tends to be male rather than female, in the same way as there are no exact female equivalents of Casanova, Don Juan or the word womanizer in English. Femme fatale might be close enough, although it implies that the woman represents a danger for men. No such think with the vegetable.
Grammatically speaking, a cœur d'artichaut is something you may have, as in tu as un cœur d’artichaut, (you have an artichoke heart) or you may be, as in tu es un cœur d’artichaut (you are an artichoke heart.)
This idiom dates back to the 19th century, and is built as a variation on the maxim: Cœur d’artichaut, une feuille pour tout le monde - Artichoke heart, a leaf for everyone.
It plays on the fact that the soft center of the artichoke is called its heart, making it natural to link it to matters of love, and suggests that each of its many leaves represents a different romantic interest.
Elle était très amoureuse de lui, mais elle s’est vite rendu compte que c’était un cœur d’artichaut.
She was very much in love with him, but she soon realized he was fickle in love.
C'est un gentil garçon, mais il semble incapable d'avoir une relation amoureuse durable. Un vrai cœur d'artichaut!
He is a nice guy but he seems unable to build a long-term love relationship. He's a ladies' man!