In a disagreeable, unpleasant, or offensive manner; in a way that causes discomfort or displeasure.
From disagreeable + -ly (adverb-forming suffix). Standard adverbial form that has been in use since the 17th century.
Disagreeably is the kind of word that sounds prim when spoken—'The room smelled disagreeably of cabbage' or 'She spoke disagreeably to the servant'—which is probably why it appears so often in Victorian novels.
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