Some sources have cited mooning, or baring
one’s butt at another as an insult that stretches back to the
Romans, but the gesture as we know it today seems to have started in
the Middle Ages.
Wikipedia claims that the first known instance of mooning was
recorded by the famous Roman-Jewish historian Josephus in the 1st
century A.D. According to Josephus’ account in The Wars of the Jews,
a Roman soldier bared his rear to an audience of Jews celebrating
Passover, and incited a riot that killed “upwards of thirty
thousand.” However, a closer examination of Josephus’s account shows
that the soldier was not mooning the crowd, but rather farting in
their general direction. Josephus puts it more delicately, “One of
the soldiers, raising his robe, stooped in an indecent attitude, so
as to turn his backside to the Jews, and made a noise in keeping
with his posture.”
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 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."
Though it was a worldwide phenomenon by the 19th century, mooning
didn’t get its name until the 1960s. The Oxford English Dictionary
dates moon and mooning to student slang of the 1960s, when the
gesture became increasingly popular at American universities. The
term derives from the use of moon or moons as slang for the bare
buttocks.