The word "stumble" generally refers to the action of tripping or losing one's balance while walking or running. It can also mean to make an error in speech or action, often used in a more figurative sense when someone makes a mistake or has difficulty in doing something, such as remembering a line in a performance or answering a question.
The word "stumble" comes from the Middle English "stumblen," which is derived from the Old English "stumblian," meaning to trip or fall. The root is thought to relate to the idea of being unsteady on one's feet.