Origin and Application:
You may wonder how the image of an apple is related to our eyes. The apple in our eye refers to the pupil, which was believed to be as solid as an apple in the old days. The phrase, appearing five times in the Old Testament, refers to a favourite of someone.
It is generally thought that this idiom appeared first in the Bible. But according to historians, the idiom appeared as early as in the ninth century by King Alfred who translated the original Latin phrase “pupillam”, meaning “pupil”, into Old English. The image of apple was added to sound poetic. Apple of one’s eye now describes something considered as precious as one’s eyes.