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 and therefore 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', 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 used with the longbow
that the symbolic gesture is known as "flipping the bird."
And yew thought yew knew everything!