Why is an expensive, yet sometimes unproductive, gift called a "white elephant" gift? 

This allegedly originates with one of the Kings of Siam, who supposedly gave white elephants to members of his court he wished to ruin. At that time, white elephants were considered sacred and were not allowed to work, yet they still had to be fed and taken care of. As a result, a possession/gift that must be maintained at high cost but offers no productive output is called a "white elephant."