Bows and arrows used to be meade from the yew tree in the 1500s, so when an arrow was shot it was called "plucking the yew." At the Battle of Agincort, the French party was going to capture the English and cut of their middle fingers so they couldn't pluck the yew. However, the English people heard of their plan and weren't captured. So, they taunted the french people by waving their middle fingers and saysing "Look, we can still pluck yew!" So "pluck yew" was a phrase used to taunt french people, until it evolved to "fuck you" and the hand gesture was associated with... well, we all know what it's supposed to look like