When you forbid something, you refuse to allow it. You might have heard your parents say things like, "I forbid you to play ball in the house!" or "I forbid you to have any more parties when we're not home!"
Forbid comes from the Old English word forbeodan, meaning "forbid, prohibit." Forbid can also mean "prevent" or "keep from happening." The spare tire on your car will forbid you from driving any faster than 40 miles an hour — any faster and the car will shake. You may have heard forbid used in the expression "God forbid . . ." This refers to something you hope will not happen, so much so that you're asking God for a little help.