No, Mrs Duck, the erected middle finger doesn't have anything to do with archery. It is an ancient protecting gesture against evil, like crossing fingers and such things.
The erected phallus is an important symbol of ancient male nature and fertility gods. As male power is often connected with fertility, (think of the stag, a major symbol of celtic Cernunnos and germanic Frey, both fertility gods) the erected phallus seemed to be the right protective symbol against evil spirits and wizardry. As Wotan, God of death, and Freya, female goddess of fertility (and therefore something like the "Master Witch") were the entities of magic, Frey, the male symbol of life, was adequate to overwhelm both, death and femal power. For only through frey's phallus freyas fertility was to bear fruit.
The ancient romans also believed in the power of phallus symbols. In Pompeji archeologists have discovered Phalli everywhere - small statues of Priapos, Satyrs or other nature gods, amulets, even oil lamps...
The slogan "Fuck off" or "Fuck you" is truly deeply connected with the middle finger gesture... By the power of sexual intercourse you hushed evil spirits away - nowadays you just demonstrate them that they are a nuisance.