There is more than a suggestion in the choice of the middle finger, in this instance. Among the Romans, the middle finger was known as the “infamous finger.”

Infami digito et lustralibus ante salivis
Expiat, urentes oculos inhibere perita.
Persius, Sat. ii

See also Dio Chrysostom, xxxiii. “Neither,” says Lampridius, Life of Heliogabalus, “was he given to demand infamies in words when he could indicate shamelessness with his fingers,” Chapter 10. “With tears in his eyes, Cestos often complains to me, Mamurianus, of being touched by your finger. You need not use your finger, merely: take Cestos all to yourself, if nothing else is wanting in your establishment,” Martial, i, 93

To touch the posteriors lewdly with the finger, that is, the middle finger put forth and the two adjoining fingers bent down, so that the hand might form a sort of Priapus, was an obscene sign to attract catamites. That this position of the fingers was an indecent symbol is attested by numerous passages in the classical writers. “He would extend his hand, bent into an obscene posture, for them to kiss,” Suetonius, Caligula, 56. It may be added that one of that emperor’s officers assassinated him for insulting him in that manner. When this finger was thus applied it signified that the person was ready to sodomise him whom he touched. The symbol is still used by the lower orders.

“We are informed by our younger companions that gentlemen given to sodomitical practices are in the habit of frequenting some public place, such as the Pillars of the County Fire Office, Regent St., and placing their hands behind them, raising their fingers in a suggestive manner similar to that mentioned by our epigrammatist. Should any gentleman place himself near enough to have his person touched by the playful fingers of the pleasure-seeker, and evince no repugnance, the latter turns around and, after a short conversation, the bargain is struck. In this epigram, however, Martial threatens the eye and not the anus.” The Romans used to point out sodomites and catamites by thus holding out the middle finger, and so it was used as well in ridicule (or chaff, as we say) as to denote infamy in the persons who were given to these practices.

“If anyone calls you a catamite, Sextillus,” says Martial, ii, 28, “return the compliment and hold out your middle finger to him.” According to Ramiresius, this custom was still common in the Spain of his day (1600), and it still persists in Spanish and Italian countries, as well as in their colonies. This position of the fingers was supposed to represent the buttocks with a priapus inserted up the fundament; it was called “Iliga,” by the Spaniards. From this comes the ancient custom of suspending little priapi from boys’ necks to avert the evil eye.

Aristophanes, in the “Clouds,” says:

SOCRATES: First they will help you to be pleasant in company, and to know what is meant by OEnoplian rhythm and what by the Dactylic.

STREPSIADES: Of the Dactyl (finger)? I know that quite well.

SOCRATES: What is it then?

STREPSIADES: Why, ‘tis this finger; formerly, when a child, I used this one.

(Daktulos means, of course, both Dactyl (name of a metrical foot) and finger. Strepsiades presents his middle finger with the other fingers and thumb bent under in an indecent gesture meant to suggest the penis and testicles. It was for this reason that the Romans called this finger the “unseemly finger.”)

SOCRATES: You are as low minded as you are stupid.

[See also Suetonius. Tiberius, chapter 68.]