What Does The Phrase 'White Elephant' Mean?

A white elephant is a term which is used to mean an expensive but useless item.
It originates from ancient Siam, (present day Thailand).  Whenever a white elphant was captured, it immediately became the property of the king.  And it was unlawful to kill it or harm it.
The king used to present the white elephant to someone who had angered him.  The elephant was expensive in terms of its upkeep, but had to be kept in good condition.  It was also unlawful to kill it.  Consequently, the new owner regarded the white elephant as a direct route to financial impecunity.
The term is still used today.  the last famous 'white elephant' in the UK was generally thought to be the Millennium Dome, which was built in Greenwich to celebrate the Millennium, but was never the success it was hoped that it would be ; despite the enormous cost of building it!