We saw people mooning in the movies Brave and Braveheart, which make it seem like an ancient Scottish ritual, although adults might assume that Brave (2012) took the idea from Braveheart (1995). Still, you have to wonder whether Scots really did that, and were they the ones who came up with the idea. The answer is, um, no. Baring one's butt as an insult to one's enemies has a long and sordid history, that may have extended back before written language. However, the existing literature does have quite a few examples.
There is ample evidence that people mooned each other during the Middle Ages. One of the earliest known instances of mooning happened during the Fourth Crusade around 1203, when Western Europeans attempted to take Constantinople. As the crusaders’ ships pulled away after the failed attack, the Byzantines hooted and hollered and “showed their bare buttocks in derision to the fleeing foe.” Another account tells of the Italian nobleman and troubadour Alberico da Romano, who in the first half of the 13th century was so indignant at losing his favorite falcon during a hunt that he “dropped his trousers and exposed his rear to the Lord as a sign of abuse and reviling.”
They didn't call it "mooning" in the Middle Ages, so you have to wonder if the common people had their own term for the gesture, if it was that common. Read what we know about the history of mooning, in a link-heavy article at Slate. -via Metafilter