Entry: unbent (adj.)
In context: "She was unbent where that dog was concerned, I remember."
Definition: None of the OED's definitions fit here. It seems, given the context, that DFW was indicating an unhealthy emotional/mental attitude towards the dog. Which is odd, because bent (adj.) can be slang for the same thing.
Anyway, here are the more traditional meanings:
Not bowed or curved; also, freed from bending, straightened.
Not subdued or made subservient.
SNOOT score: 1
Source: Oxford English Dictionary