someone or something cherished or highly valued
—He can’t stop smiling when he sees his baby daughter. She’s the apple of his eye.

It seems that the pupil of the eye was once thought to be a solid sphere, like a round fruit. And apple was once a general name for fruits of many kinds. Therefore, long ago, apple was another name for the pupil.

Speaking about God’s love for Jacob (Israel), Moses said,

For the Lord’s allotment is his people,

Jacob is his special possession.

The Lord found him in a desolate land,

in an empty wasteland where animals howl.

He continually guarded him and taught him;

he continually protected him like the pupil of his eye [the apple of his eye—KJV]. (Deuteronomy 32:9,10)

The original Hebrew version of this phrase can be translated literally as “little man in the eye,” probably referring to the small reflection a person can see of himself when looking into someone else’s eye. Pupil comes from a Latin word with the similar meaning of “little doll.”