A riposte is a clever comeback or witty response. Many people can only think of the perfect riposte after the conversation has already moved on, when it's too late to say it!
Winston Churchill was famous for his skill with a riposte, like when his friend Lady Astor said that if he were her husband, she'd poison his tea, and he responded, "If you were my wife, I'd drink it!" This kind of quick, funny retort is a classic riposte. Originally, the word riposte came from a French word for a certain kind of lunge in fencing, and it still has this same meaning today.