So, it turns out that M.I.A. made a particular unidigital gesture during a Super Bowl performance. Cue apologies similar to those that followed a wardrobe malfunction on a previous show. The BBC report wonders how old the gesture might be and points to Diogenes the Cynic. Diogenes Laertius (6.34) reports:
ξένων δέ ποτε θεάσασθαι θελόντων Δημοσθένην, τὸν μέσον δάκτυλον ἐκτείνας, “οὗτος ὑμῖν,” ἔφη,“ἐστὶν ὁ Ἀθηναίων δημαγωγός.
When some friends from out of town wanted to do and see Demosthenes, he stretched out his middle finger and said, "This is the Athenians' demagogue".
It seems that there was little love lost between Diogenes and Demosthenes. And Diogenes is not averse to a bit of shocking behaviour so perhaps it is a gesture of contempt. But I wasn't not sure whether what he have here is an insulting gesture of contempt aimed at his guest-friends for wanting to go and see Demosthenes in action.
A similar story appears in Epictetus 3.2.11:
οὐκ οἶδας, ὅτι Διογένης τῶν σοφιστῶν τινα οὕτως ἔδειξεν ἐκτείνας τὸν μέσον δάκτυλον, εἶτα ἐκμανέντος αὐτοῦ ‘Οὗτός ἐστιν’, ἔφη, ‘ὁ δεῖνα· ἔδειξα ὑμῖν αὐτόν’;
Anyway, a quick TLG search seems to confirm that it is more or less the gesture we know now. Scholiasts on Aristophanes Clouds (ad 653 and 549) suggest as much. And here is the Suda:
The verb is σκιμᾱλίζω,'to hold up the middle finger':
τῷ μέσῳ δακτύλῳ συναρμόσας τὸν μέγαν καὶ πλήξας ἐφυβρίζει. ἢ ἀντὶ τοῦ κατεδακτύλισε· σκιμαλίσαι γάρ ἐστι κυρίως τὸ μέσον τὸν δάκτυλον εἰς τὸν πρωκτὸν τοῦ ὀρνέου ἐμβαλεῖν. οὐ μόνον δὲ τοῦτο, ἀλλὰ καὶ ὅταν βουλόμενοι ἐνυβρίσαι τινά, τὸν μέσον δάκτυλον ἐντείνοντες καὶ τοὺς λοιποὺς συνάγοντες δείξωσιν αὐτῷ. Ἀριστοφάνης· καὶ τὸν δορυξὸν οἷον ἐσκιμάλισεν.
So, it is principally to do with shoving a middle finger up a bird's anus and also can be used to insult someone.