Excessively or more than is necessary, proper, or reasonable; unjustifiably.
From 'un-' (not) plus 'duly' (from Latin 'debitus' meaning owed or proper). It combines to mean 'not properly' or 'excessively.'
The word 'duly' (meaning properly or appropriately) is much rarer than 'unduly,' which shows how English often uses negative forms more frequently than their positive counterparts in actual speech.
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