A rejoinder usually means a witty comeback. If someone asks you a silly question like, "Are you painting?" when you are holding a paint can and a brush, your rejoinder could be, "No, I'm just doing my nails."
The word rejoinder comes from the Middle French word rejoindre, which meant to answer a legal charge. The meaning of a witty retort developed from the legal meaning. You often think of the best rejoinders long after the crucial moments have passed, which can be really frustrating.