The Hebrew noun ishon (אִישׁוֹן) has two related meanings: (1) the pupil of the eye — literally the small dark opening at the center of the iris, and (2) deep darkness — the blackness of midnight. The word may derive from ish (man, H376), as the pupil was understood as a tiny 'little man' (the miniature reflection of a person visible when looking into another's eye).
Metaphorically, ishon is used for something precious and carefully protected — the most sensitive and vital part of vision, easily damaged and requiring the greatest care.
The phrase 'apple of the eye' (literally 'pupil of the eye,' ishon bat-ayin) is one of the most tender expressions of divine protection in the Old Testament. Deuteronomy 32:10 describes God guarding Israel like 'the apple of his eye' — the most precious, carefully protected part of a person. Zechariah 2:8 uses the same imagery with eschatological urgency: 'whoever touches you touches the apple of his eye.'
The intimacy of this image is profound. The pupil is both the organ of sight and the most vulnerable point of the body — a tiny wound to the cornea causes blinding pain and vision loss. To call someone the pupil of God's eye is to declare that God guards them with the reflexive, instinctive protectiveness of a person guarding their own sight. This is the depth of God's protective love for His people — covenantal, urgent, and fiercely personal.