For the benefit of our American friends, "roundly" comes from Middle English (roundly, roundely, or roundliche), formed by adding the adverb suffix -ly to the adjective round. It has been in use as an adverb since at least the 15th century. In modern English (especially from the 19th century onward), "roundly" became a common intensifier in phrases like 'roundly condemned', 'roundly defeated' or 'roundly disliked' - the latter conveying that the negative opinion is unanimous, emphatic, or shared by almost everyone - not just mild or partial dislike. Thus, the polite, quintessentially English phrase 'roundly disliked' is a classic example of British understatement - essentially a euphamism for 'fucking hate the cunt'.