In the battle of Agincourt, the French, who were overwhelmingly favored to win
the battle, threatened to cut a certain body part off all captured English
Soldiers, so they could never fight again.

The English won in a major upset and waved the body part in question at the
French in defiance. The body part that the French proposed to cut off the
English after defeating them was, of course, the middle finger, without which
it is impossible to draw the renowned English long bow. This famous weapon was
made from the native English yew tree, and so the act of drawing the long bow
was known as "plucking the yew". Thus when the victorious English waved their
middle fingers at the defeated French, they said, "See, we can still pluck yew!
PLUCK YEW!"

Over the years some "folk etymologies" have grown up around this symbolic
gesture. Since "pluck yew" is rather difficult to say like "pleasant mother
pheasant plucker", which is who you had to go to for the feathers used on
arrows. The difficult consonant cluster at the beginning has gradually changed
to a labiodental frictive "F", and thus the word often used in conjunction with
the one-fingered-salute is mistakenly thought to have something to do with an
intimate encounter. It is also because of the pheasant feathers on the arrow
that the symbolistic gesture is know as "giving the bird".

AND YEW THOUGHT YEW KNEW EVERYTHING...