In a manner resembling or characteristic of a dandy; with excessive attention to fashion and appearance.
From dandy + -illy (a variant of -ily, the adverb-forming suffix). This is a somewhat archaic or whimsical variant of the more common 'dandily,' showing how English speakers experimented with different ways to turn adjectives into adverbs.
Dandilly is like a ghost word—it appears in old texts but sounds old-fashioned even now, while dandily (the -i- version) became the standard. This shows how English constantly prunes unusual variants in favor of the clearest, most efficient forms.
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