The book had been read until it was falling apart, but he couldn't bear to throw it out.
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history
From the late-Qing satirical novel 《官场现形记》 by Li Baojia (ch. 20): officials affecting poverty wore clothes 极破烂不堪. The suffix 不堪 (unbearably) piles on degree - 'ragged to the point of unbearable.'
grammar
不堪 acts as an intensifying suffix meaning 'unbearably / to an extreme degree.' Same pattern: 疲惫不堪 (utterly exhausted), 狼狈不堪 (in a miserable state), 拥挤不堪 (unbearably crowded).
