to reiterate that this is humour and i haven't checked the actual history,
so don't take it too seriously in case poetic license is involved...
The History of Giving Someone the Middle Finger
(aka: Fliping the Bird)
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 they would be
incapable of fighting in the future.
This famous English longbow 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!"
Since 'pluck yew' is rather difficult to say, the difficult consonant cluster
at the beginning has gradually changed to a labiodentals fricative 'F', and
thus the words often used in conjunction with the one-finger-salute! 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.'