An archaic or obsolete adjective meaning difficult, steep, or hard to accomplish.
From Latin 'arduus' meaning 'steep' or 'difficult,' possibly related to 'ardere' (to burn) in the sense of being 'hard to climb' or 'burning hot.' This word is now rarely used in English except in historical texts.
The Latin word 'arduus' originally described steep mountains that were hard to climb, but medieval writers started using it for any difficult task—showing how physical difficulty became metaphorical difficulty.
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