very serious, severe, or causing great harm or suffering.
From Old French 'grevous', from Latin 'gravis' meaning 'heavy.' The 'heaviness' metaphorically became seriousness.
This word appears in the famous legal phrase 'grievous bodily harm' (GBH) in British law, and the adjective is so dramatic that it's rarely used in casual speech—people say 'serious' instead!
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