, a "bone
s" file is a file that contains a dump
of the level
some hapless character died on.
What makes it so bony is that it contains all (or most) items the character had (most cursed, though), and the grave for the unlucky adventurer's earthly remains. Or, perharps, a ghost of the character (or, if the character was killed by a vampire or a zombie, something similiar). And the pets, who may be a bit hungry after wandering around the level for a long time without food!
Some later adventurer may stumble on the very same level when the bones file is loaded; the file is usually then removed. This will probably give you more willingness to play again if you get an YASD, and will probably give a lot more excitement if there are others who are playing on the same machine!
The problem with bones files is that they aren't compatible between different versions, and as a matter of the fact, they're not even compatible between different builds of the same version on the same machine!
Bones files are exteremely interesting when the game is played on servers by many players. The Windows users probably want to check out Hearse, a tool that transfers compatible bones files between many users.
Craziest thing that ever happened to me: I died twice on the same level, and the game generated bones file on the second time too! I had some really juicy junk down there (couple of GDSMs, and artifacts)... too bad this version incompatibility thing ruined the fun. =(
See: Hearse, Nethack, Nethack Meta Node