Yippee is the happy shout you let out when something wonderful happens — school is cancelled, the trip is on, you won. It is not a word inside a sentence; it stands alone, usually with an exclamation mark: "Yippee!" Or it opens one: "Yippee, we're going to the beach!" The feeling is bright, jumping-up-and-down joy, and that is exactly why grown-ups use it carefully: it sounds like a child, or like a cartoon. So a small child saying it is completely natural, and an adult saying it is usually playing, being warm with children, or being deliberately silly. That leads to its other common use, which you must be able to hear: said flatly, with no energy, "yippee" means the opposite — "how wonderful" said in a bored, unimpressed voice. "Another two hours of meetings. Yippee." The words are the same; the tone carries everything. In writing, people show the fake version by putting it after bad news, or by writing "yippee" with no exclamation mark. You may also meet the cowboy shout "yippee-ki-yay", which is a joke reference to an old film and a different thing again.