In a manner of speaking or writing at length; in an elaborating or expansive way.
From expatiating (present participle of expatiate) plus the adverb suffix '-ly'. A rare literary adverb formed from the verb form.
This adverb is almost certainly a rare creation—the kind of word a Victorian novelist might use to describe how a character speaks with elaborate enthusiasm and detail. English allows such combinations, but some feel more 'natural' than others.
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