In my current reading of The Power and the Glory, the word ‘darn’ came up outside of it’s usual usages as an interjection or substitute for damn, unless Miss Lehr has something against socks. In relation to clothing, ‘darn‘ has a different meaning:

darn
verb
To mend (a garment, for example) by weaving thread or yarn across a gap or hole.
noun
A hole repaired by weaving thread or yarn across it.