As to the percussion-caps, he had almost resolved, in the bitterness of his heart, that he would throw them into a neighboring pond; for not only was he the solitary pupil, but he began even to have a certain scepticism about guns, and a general sense that his theory of life was undermined.
There are no more uses of "undermine" in the book.
Show samples from other sources
Her father had a way of undermining his sister’s lectures on the innate superiority of any given Finch: he always told his daughter the rest of it, quietly and solemnly, but Jean Louise sometimes thought she detected an unmistakably profane glint in Atticus Finch’s eyes, or was it merely the light hitting his glasses?
Harper Lee -- Go Set a Watchman
He cocked his head, trying on his "I’m so wholesome" routine, but his bloodshot eyes undermined his efforts.