The phrase "basket case" originally referred to a person who was physically disabled to the extent of requiring a basket to be carried. Its origin dates back to World War I, where it was used to describe soldiers who had lost limbs and had to be carried in a basket. 

Over time, the phrase evolved to take on a more figurative meaning. Today, "basket case" is commonly used to describe someone who is mentally or emotionally unstable or unable to cope with everyday life. The transformation from its literal wartime meaning to a broader, more metaphorical usage reflects the way language adapts and evolves over time.