In a manner that displeases or causes displeasure.
From displeasing (adjective) plus the adverb suffix -ly. This is a standard formation that stacks multiple suffixes to build meaning.
English lets us stack suffixes almost endlessly—'displease' becomes 'displeasing' becomes 'displeasingly'—and speakers instantly understand each layer, which is why English is flexible but also sometimes has 20-letter words!
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