Hasn’t everybody heard? The bird is the word.
From a 1964 song written by The Trashmen called “Surfin’ Bird.”
No doubt made popular by the episode in Family Guy in which Peter sings and dances to part of the song.
Here’s the original.
Oh, it seems some people think that the bird (finger) is the F-word and that is what this refers to. But that does not hold much weight.