How did the expression 'apple of my eye' come about?

The term was originally used in The Bible and it was used to signify someone of great worth 'whoever touches you touches the Apple of his eye', someone who was very precious. The 'Apple of your eye' is the iris or center of the eye. If someone were to touch the Apple of your eye it would be very painful and so it is protected by the eyelid. The biblical imagery is that we are the Apple of his eye and God is the eyelid who protects us. Shakespeare also used the term with this meaning in 'A Midsummer Night's Dream' to again show something as most precious -

Flower of this purple dye,

Hit with Cupid's archery,

Sink in Apple of his eye

Modern usage of this idiom usually refers to something that is especially beloved.

The saying originally meant the central aperture of the eye, presumably because Apples were the most common sphere-shaped object around. The Apple and Apple tree were also a sacred symbols for the early British peoples. This is reflected in such stories as the Island of Avalon (which literally means Apple Island). Now, this phrase is usually figurative, meaning something, or more usually someone, cherished above others. As sight is so precious, someone who is called this as an endearment is similarly precious.