> In Greek the gesture was known as the katapygon (κατάπυγον, from kata – κατά, "downwards" and pugē – πυγή, "rump, buttocks"). In ancient Greek comedy, the finger was a gesture of insult toward another person, with the term katapugon also referring to "a male who submits to anal penetration" or katapygaina to a female. In Aristophanes's comedy The Clouds (423 BC), when the character Socrates is quizzing his student on poetic meters, Strepsiades declares that he knows quite well what a dactyl is, and gives the finger. The gesture is a visual pun on the two meanings of the Greek word daktylos, both "finger" and the rhythmic measure composed of a long syllable and two short, like the joints of a finger (— ‿ ‿, which also appears as a visual pun on the penis and testicles in a medieval Latin text).

From here: https://wikiless.org/wiki/The_finger?lang=en#Classical_era