Exhausted or worn out from grueling exertion; severely tested or harassed.
From 'gruel' (thin, watery porridge—meager sustenance) which became a verb meaning 'to exhaust,' plus past participle '-ed'. The image is of being reduced to one's minimum by harsh treatment.
It's poetic that 'grueled' comes from 'gruel'—thin, barely-there food—because the verb means to reduce someone to their bare minimum through exhaustion. The etymology literally matches the meaning!
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