Lookups for disposition have been rising steadily for several months, and hit the #1 spot on March 4, 2011.
The popularity of this word got a boost from "Sweet Disposition," a song by the Australian band The Temper Trap. That song appeared in commercials during the recent Academy Awards broadcast, and a remix was nominated for a Grammy.
In the song, disposition is used to mean "the usual attitude or mood of a person."
In other less-romantic contexts, the word's meanings include "the order or arrangement of something," and "the official control or power of transferring property to another person."
Disposition comes from a Latin word that means "to put in order, to arrange."