This started its life, at the end of the eighteenth century, as a positive word that was associated with the qualities of good public speaking, especially a way of speaking that was full, round and imposing. It comes from the Latin phrase ore rotundo, “with rounded mouth” which appears in Ars Poetica by the Roman writer Horace (so the word has a very close link to rotund, which comes from Latin rotundus, as indeed does round, all three ultimately deriving from rota, a wheel.)
In 1840, the Penny Cyclopaedia described the qualities of such speech in terms themselves as orotund as one could reasonably wish: “The name of orotund ... is given to that natural or improved manner of uttering the elements, which exhibits them with a fulness, clearness, strength, smoothness, and a ringing or musical quality rarely heard in ordinary speech”. One thinks of rounded vowels rumbling up from rounded bellies, of expansive language accompanied by equally expansive gestures.
But such oratorial tendencies can quickly tip over into pretentious or pompous long-windedness, and this is the word’s downside. When William Makepeace Thackeray wrote The Book of Snobs, which appeared in 1848, he had this other sense in mind, but preferred the Latin original to the derived English term:
Jawkins is a most pertinacious Club Snob. Every day he is at that fireplace, holding that Standard, of which he reads up the leading-article, and pours it out ore rotundo, with the most astonishing composure, in the face of his neighbour, who has just read every word of it in the paper.