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!
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 for the longbow),
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.