My favorite middle-finger related joke is that it's the state bird of New York. Ideally, you introduce it with a bit of related ornithology trivia.

Some (modern) explanations, as well as an interesting report about hand gestures around the world, are included in The Independent (Nov 7, 1992)

I don't have access to it, but this chapter of A History of Cant and Slang Dictionaries: Volume III: 1859-1936 pulled up a promising preview with the word "gesture"

Alternatively, it could be the "thumbs up." Only problem is that it's one finger. From https://www.army.mil/article/197973/echoes_of_world_war_i

thumbs up -- A phrase or gesture that everything is fine. May derive from the Roman gesture to spare a downed gladiator in the arena. Now mostly an American gesture, it must be used carefully as it has more vulgar meanings in other cultures [ie, "up yours"].