It is a fine historic English gesture attributed to the English archers at the battle of Agincourt in 1415 who the French had threatened to cut the bow fingers from, after the battle the fine Victorious English Archers showed the vanquished enemy their remaining digits and said F#ck off you French C###s The meaning and gesture has stayed the same ever since
I always thought that was the origin too but on an episode of QI Stephen Fry claimed it was just an urban myth. Can’t remember for the life of me what the true origin was though so I’ll just agree with what you’ve said for now