The Apple Of One’s Eye is somebody (usually a child) who is regarded as precious and irreplaceable.
The earliest appearance of the term is found in King Alfred’s writing in the ninth century. Originally this term simply referred to the “aperture at the centre of the human eye” viz. the pupil. This appears to be the meaning Shakespeare used in his 1590s play A Midsummer Night’s Dream. In the play, the fairy character Robin Goodfellow has acquired a flower that was once hit by Cupid’s arrow, imbuing it with magical love-arousing properties, and drops juice of this flower into a young sleeping man’s eyes, saying
“Flower of this purple dye, / Hit with Cupid’s archery, / Sink in apple of his eye”.
It also appears in the King James Bible translation from 1611:
Deuteronomy 32:10: “He found him in a desert land, and in the waste howling wilderness; he led him about, he instructed him, he kept him as the apple of his eye”.
Some interpret the idiom as a adaptation of the Latin word, pupilla, which means a little doll, and is a diminutive form of pupus, boy, or pupa, girl (the source also for our other sense of pupil to mean a schoolchild.) It was applied to the dark central portion of the eye within the iris because of the tiny image of oneself, like a puppet or marionette, that one can see when looking into another person’s eye
Sight was regarded as the most valued of all the senses and therefore the ‘apple’ was precious and irreplaceable.