An interesting bit of history from Chaos Manor:
Subject: Plucking the Yew! Before the Battle of Agincourt in 1415, the French, anticipating victory over the English, proposed to cut off the middle finger of all captured soldiers. Without the middle finger it would be impossible for the English soldiers to draw the renowned English longbow and therefore incapable of fighting in the future. The famous bow was made of the English Yew tree and the act of drawing the longbow was known as “plucking the yew” or “pluck you”. Much to the bewilderment of the French, the English won the battle and began mocking the French by waving their middle fingers at the defeated French and saying “We can still pluck yew. Pluck you”. 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. 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”. And yew thought that yew knew everything.The full story here.