In a manner that defaces or damages appearance; in a way that spoils or mars something.
From defacing + -ly (adverb-forming suffix). English word formation creating an adverb from the gerund/present participle form of a verb.
Defacingly is so rare that it's almost never used—English speakers just say 'in a defacing way' instead, showing that not every possible grammatical combination actually survives in real language.
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