Could it be a swear? Like "blood and thunder" or "Thralls balls!"
Is he referring to actual bones? Does he have like a bone shield or something that Voss destroys? This would provide reasoning for why he flees yet again instead of fighting us right there.
Is he referring to her own bones, which the players make unusable by beating the tar out of Voss?
The way the voice actor says the line leads me to believe he's referring to actual bones, but I've noticed none in the area or as a buff on him. Just seemed like a strange bit of dialogue, and I'd be interested to know what others thought about it.
edited for grammar.