Un chaud lapin

Literally: A hot rabbit

Example: “Méfie-toi de ton nouveau collègue, c'est un chaud lapin.” — Watch out for your new colleague; he's always chasing skirts.

English equivalent: To chase skirts

Origin: Built on the rabbit's genuinely remarkable arithmetic. A doe carries a litter about 31 days, can bear up to a dozen young, and can be fertilized again within roughly a day of giving birth — which a buck sharing the hutch will not decline. Chaud carries the sense used of animals in rut, and the phrase simply transfers that appetite to a man. A note on the word itself: French called the animal connil or connin from Latin cuniculus, which is why rabbit farming is still cuniculture; lapin only takes over from the 16th century.