The History of the Middle Finger.

Before the Battle of Agincourt in 1415, the French, antici-
pating 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.'

[I don't believe a word of this, but it's still a fun read.] 

Source: 

http://laffaday.gophercentral.com