Throughout the novels, there are several references to "The Bones." Typically, this is a common expression of luck, as in the phrase "have a good roll of the Bones," or an expression of fatalism, as in the phrase "as the Bones would have it." While most people in these stories are not fatalistic, the terminology has been so completely engrained in the vernacular that hardly anyone thinks about it. Few people would be able to explain the origins of the terms, and hardly anyone would think there was any real significance behind it.
It's a reference to an ancient but no longer practiced form of divination, typically derived from the tossing of two (but sometimes more) six-sided dice made of bone (but sometimes wood, stone, or ivory). The six faces were carved with images of 1) a single bone, 2) two empty eye sockets, 3) a pelvis, 4) a set of ribs, 5) a skeletal hand, and 6) a skull.
While various dice have been discovered in tombs, washed up on coastlines, or even inside the forgotten vaults of kings, the meanings of these pieces has been lost. Apparently the use and art of "reading" the Bones was never recorded and has been lost to time. It survives only in catch phrases now devoid of original meaning. Even so, the most formal of sects forbid speaking of the Bones, believing there exists inherent danger in referencing even a defunct practice. Agnostics, on the other hand, use the term quite prevalently, almost to the point of the Bones becoming their focus of worship, which they would ardently deny.
In the ancient divinations, rolling a pair of "skulls" (or sixes) was considered the most favorable outcome. Rolling a pair of "crossed bones" (or ones) was considered the least favorable.
Today, six-sided dice are commonplace in taverns as tools of gambling or for playing games of chance. The images have been replaced with either numerals or an equivalent number of pips, occasionally with the "six" side still featuring an image of a skull and the "one" side showing a "bone."