Extremely tiring and demanding; exhausting in a way that tests your physical or mental limits.
From the verb 'gruel' (to exhaust), which comes from the noun 'gruel'—a thin, watery porridge given to poor people and prisoners. The British spelling 'gruelling' reflects the doubling of 'l' when adding '-ing' to a stressed syllable. The idea is that enduring something gruelling is like surviving on thin gruel.
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