In a manner that is witty or humorous but presented without emotion or expression, like making a joke deadpan.
From 'dry' plus the adverbial suffix '-ly.' The sense of 'dry' as meaning 'dull' or 'lacking embellishment' evolved to mean 'without emotional expression,' making 'dryly' the way you deliver a joke with a straight face.
Dry humor is sometimes called British humor because it's associated with understatement and restraint—when someone says something hilarious but in the flattest possible voice, that's dryly delivered, and it's often funnier precisely because it seems so casual.
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