White elephants have been regarded as holy in Thailand, India and other Asian countries since ancient times. The owner of a white elephant was required by law to pay for the upkeep of the elephant with special (and thus, expensive) food and to provide access for common people to worship the white elephant.
It is said that if a Thai king was dissatisfied with a subject, he would give the subject a white elephant. Since the elephant was holy, it could not be refused. As a consequence, the cost of maintaining the elephant would bankrupt the subject, reducing him to poverty and suffering. Thus, the "gift" was a gift meant to hurt the recipient.
Idiomatically, a white elephant is something that is very expensive to maintain, and which provides absolutely no benefit whatsoever to the owner.
Example: The Millennium Dome, a structure built in England to house an exhibition from January 1, 2000 until the end of that year (which brought in the new millennium, hence the name). The Dome is considered to be a poorly planned and poorly managed project that did not attract the expected number of visitors. Also, after the original exhibition was closed, the Dome cost one million pounds per month to maintain. Because of this, many considered the Dome to be a white elephant, a building that was nothing but a financial burden to the government, and to taxpayers.
The English word boondoggle is often used with a similar meaning, implying financial waste without meaningful gain.