A retort is a short, clever response to someone's comment or question. If you want to keep the peace during dinner, you should probably bite your tongue instead of making sarcastic retorts to everything your little sister says.
Today retort is used as both a noun and a verb, and both come from 16th- and 17th-century sources meaning "to twist or turn back." To retort is to make a comeback, or a quick, witty answer or remark. It can be a form of payback when someone tries to insult or tease another person, and often, the perfect retort doesn't occur to you till later, when you think "Oh! I wish I had said that earlier!"