To cripple is to leave someone unable to walk. Terrible car accidents, for example, can sometimes cripple their victims.
Use the verb cripple to describe what is done to a person whose legs are so badly injured that they can't be used. You can also use the word in a figurative way, as when a parent's criticism and high expectations cripple a child's self esteem. In other words, they take away his strength and make him feel useless and unsuccessful. In the thirteenth century, cripple meant "to move slowly," from the Old English root crypel, "to bend."