Ça en jette !

Literally: It throws some!

Example: “Avec ses lustres en cristal, cette salle de bal, ça en jette !” — With its crystal chandeliers, this ballroom will knock your socks off!

English equivalent: To knock someone's socks off

Origin: A clipping of jeter un jus or jeter son jus, from the start of the 20th century, meaning to cut a fine figure. The jus is the puzzling part, and Gaston Esnault's reading is that it carries the idea of essence or extract — the best of a thing, the way the juice of a leg of lamb is — so that someone dressing to dazzle is showing the best of himself, or at least trying to make it look that way, and throwing it in the face of everyone watching. The sense started out confined to a person's appearance and widened later to anything striking enough to stop you.