And that sign was used to mean “peace” here in the States in the 60s and 70s. (Which may have caused some confusion for our British guests.)
There are a few contemporary cites for this.
For example: Thomas of Elmham was a scribe who (apparently) noted the content of Henry’s pre-Agincourt speech: his account is in Gesta Henrici Quinti, and *Elhami Liber Metricus de Henrico Quinto *in C.A. Cole (ed.), *Memorials of Henry the Fifth *(London 1858).
According to British longbow historian Robert Hardy, Elmham records Henry telling his troops of the French boast to cut off the fingers of English archers.
You’d need to check the records properly - one of my first posts on SDMB was a link to some more detail, but I think the board has swallowed stuff from 2001 (or it’s still being transferred).
I love that we got to post 13 before someone called it “the bird.”
Well, everyone knows about the bird… 
My personal theory, with no science, or learning to back it up is as follows…
Before toilet paper, back in the poop flinging monkey days of our evolution, we used our hand to wipe our backside. As I’m pissed off if someone interrupts my morning constitutional, if someone was to interrupt said constitutional, I might want to throw some poop at them.
As I’ve just been using my hand to clean off the poop, and as my middle finger is longest, it’s the most likely to have the aforementioned poop on it. I’d then simply “flick” the middle finger out towards him, hopefully, giving him a nice little bit of poop squarely flicked to the tip of his nose.
I’m sure some scientist, or historian will come along with their “proven theory”, but I’m still not going to believe them.
That’s my story, and I’m flicking to it.
Thanks for the responses. I looked at the abstract for the academic article and had to stifle a giggle at the term “fuck finger”. Heh.
Nope. It’s still available.
http://boards.straightdope.com/sdmb/showpost.php?p=2479372&postcount=12
which was in the previous thread about this same topic.
http://boards.straightdope.com/sdmb/showthread.php?t=136975&highlight=archers
I don’t have anything to add to the thread, except that I have a blog that is, shall I say – on-topic?