The French, who were overwhelmingly favored to win the battle, threatened to cut a certain body part off of all captured English soldiers so that 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 puzzler was: What was this body part?
The body part which the French proposed to cut off of the English after defeating them was, of course, the middle finger, without which it is impossible to draw the renowned English longbow.
This famous weapon was made of the native English yew tree, and so the act of drawing the longbow was known as "plucking 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 the arrows), the difficult consonant cluster at the beginning has gradually changed to a labiodental fricative 'f', and thus the words often used in conjunction with the one-finger-salute are mistakenly thought to have something to do with an intimate encounter. It is also because of the pheasant feathers on the arrows that the symbolic gesture is known as "giving the bird".
And yew all thought yew knew everything!
