The History of "The Finger"
Before the Battle of Agincourt in 1415, the French, anticipating
victory over the English, proposed to cut off the middle finger of all
captured English soldiers. Without the middle finger it would be
impossible to draw the renowned English longbow. English soldiers,
therefore, would be incapable of fighting in the future.
This famous weapon was made of the native English Yew tree, and the
act of drawing the longbow was known as "plucking the yew" (or "pluck
yew").
Much to the bewilderment of the French, the English won a major
upset and began mocking the French by waving their middle fingers at the defeated French, saying, "See, we can still pluck yew! "PLUCK YEW!"
Since 'pluck yew' is rather difficult to say, the difficult
consonant cluster at the beginning has gradually changed to a labiodental fricative 'F' (Yes, you can look it up).
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 used with the longbow that the symbolic gesture is known as "giving the bird".
And *yew* thought yew knew everything!!??