Literary or formal term for physical or mental weakness; rarely used in everyday speech.
With his weak physique, he probably cannot handle this job.
The old man is so feeble that he can barely go out.
The kanji 羸 means 'weak' or 'emaciated', and 弱 means 'weak'. The reading is on'yomi. The exact historical derivation is uncertain, but the word is a Sino-Japanese compound.