Weakness in the legs usually means one is tired or ill, according to Healthgrades. Sudden weakness in the leg can signal a much more serious condition, such as sciatica, which occurs due to pinching of the nerves in the spinal cord.
Other conditions that cause true weakness in the leg include neuromusclar diseases, such as multiple sclerosis and myasthenia gravis. Botulinum toxin can also cause the legs to feel weak suddenly. A sudden weakness in the legs can signal the beginning of a stroke, especially if it only occurs on one side of the body. If this is the case, one should seek emergency medical attention.