The bird flip?
Where did that start, what does it mean, why is it insulting?
Adding the phrase "sit & spin" answers some of these but I suspect it's a modern development, because the supposedly original gesture involved two fingers and was done by Englishmen to Frenchmen in reference to some battle where English longbow-men whooped French heavy cavalry, hence the gesture which is reminiscent of how the archers held their longbows. But this doesn’t answer where the single fingered version came from, what it symbolises or why it’s called "the bird".
Sorry I digress, insults fascinate me.
For example: Fapping idiot.
It's like "fucking idiot" except it implies the idiot in question is also prolific wanker, the word fapping being derived from fap, which is the written expression of the noise of masturbating, e.g. fap-fap-fap etc.
I suspect this came from 4chan or an equivalent.
Also notable is "drong-go" which is an Australian insult of unknown meaning, quite possibly the aboriginal word for idiot, or something like that, because nobody knows what it means it's generally used as a playful form of endearment rather than as an insult, so much so that calling one's enemy a drong-go would seem kinda strange.
We're odd like that.
I'm still doing it arn't I?