Why is the human cuspid (canine) referred to as an eye tooth?

The pointed long teeth — also called canines because they look alot like those in dogs — are called eye teeth because the pair in the upper jaw lie directly below the eyes. Originally, only the upper pair were given the name but later the pair in the lower jaw also came to be called eye teeth.

Why people seize on eye teeth as a dramatic way to indicate their longing for something is harder to get a grip on. Eye teeth are among the last of a baby’s first set of teeth to appear and so to cut them (have them emerge from the gums) implies that babyhood is in effect over. To say that somebody has cut his eye teeth means he’s wide awake and isn’t easily fooled. If you’re cutting your eye teeth (or just teeth) on something you’re gaining experience in a situation you’re new to.

These suggest that eye teeth are especially valuable, because they figuratively embody hard-learned skills and one’s experience of life. The association with eyes results in an even more powerful evocation.