“Aroynt thee, Witch, the rumpe-fed Ronyon cryes,” is the way Shakespeare wrote it in Macbeth (Act I, Scene iii), and here, as well as in King Lear (Act III, Scene iv), his obvious meaning was “Begone! Get out! Scram!” but the source of his term may never be known.
No earlier instance of the use of “Aroint” has been turned up. Nor do we know positively what he meant by a ronyon, which, in Merry Wives of Windsor (Act IV, Scene ii), he spelled runnion.
Dr. Samuel Johnson, in his dictionary of 1755, defined the latter spelling as “a mangy creature,” but defined ronion as “a fat bulky woman.”
You may take your pick. Rump, of course, pertains to the posterior of an animal.