The term originated in Asian countries, where White Elephants were considered holy but were very expensive to keep. When displeased with a subordinate, a king would give him a gift of a white elephant. The gift was so costly to keep that it would put the subordinate in financial ruin.
So the term White Elephant came to be known as a burdensome possession; creating more trouble than it is worth. A White Elephant is a valuable possession of which the owner cannot dispose and whose cost (particularly cost of upkeep) is out of proportion to its usefulness.
While that is the actual meaning it has taken on a different flavor in the many exchanges I have been part of. Yet, what it does to the people there is quite amusing. I always laugh my head off. I always am glad that I don't have to take most of the gifts home. I have also been on the receiving end of some pretty spectacular gifts, from live hamsters to science class pigs, to men's depends and buns of steel on VHS.
Part of the joy comes in the fact that it doesn't cost anything, other then the time to wrap the gift and you get to clean out your junk.
So when Brent left our house, after our small group White Elephant Gift Exchange, with 100 dollar bills taped to a ficus tree we laughed our heads off. Because we were able to clean out part of our closet.
I just know that tree will appear again at someone's house next year, like the Viagra Ball Hat I wrapped this year for someone in my group.