To lower oneself to do something one considers beneath one's dignity, usually used sarcastically.
From Old French 'deigner', from Latin 'dignari' meaning to deem worthy (from 'dignus' meaning worthy). The word entered English in the 13th century with a sense of dignity and worthiness built in.
The word 'deign' is dripping with social class—it assumes some people have dignity so high that ordinary actions would demean them, which is exactly how aristocracies justified themselves historically; our language still carries the class structure in its vocabulary.
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