This is a slightly fancy, slightly funny word for a person's buttocks — the part you sit on. English borrowed it from French, and it still feels French, which is why people use it with a little smile. You meet it in jokes, in fashion writing (a dress that shows off her derrière), and when someone wants to sound playful rather than crude. Teenagers usually just say butt or bum; magazines, comedians, and older speakers are more likely to pick derrière. It is not a doctor's word, and it is not an insult. It is the polite-looking word that is actually winking.