Causing someone pain or difficulty as a consequence for wrongdoing, or very harsh and difficult.
From Latin 'punire' meaning to inflict pain, possibly related to 'poena' meaning penalty or pain. The word evolved from literal physical punishment to include any harsh consequence.
Interestingly, 'punishing' has become a metaphor in modern English—athletes talk about a 'punishing workout' or critics give 'punishing reviews,' showing how the word expanded beyond legal punishment to mean anything severe or demanding.
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