When something is prohibited it's not allowed. If you see the sign "Swimming is prohibited," stay out of the water.
Whether used as a verb ("You prohibited your friend from entering") or an adjective ("The prohibited word escaped your lips"), prohibited means something was forbidden — a no-no. You often see it in signs like "Smoking prohibited." It's a strong word, indicating that something is banned and suggesting dire consequences if the ban is not obeyed. On the other hand, when something is prohibited it might seem more appealing. That's probably why some people think smoking and drinking are cool.