This is not meant to be crude.  It is strictly for your edification and
enjoyment.

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."  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!"

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".