The character 奶 originally meant "milk" or "breasts", associated with nursing and motherhood. To form the term for "paternal grandmother", the character is simply doubled (reduplicated), a common pattern in Chinese for creating intimate or familiar kinship terms. This repetition softens and familiarizes the single concept, transforming the foundational idea of a nourishing mother-figure into the specific, affectionate term for one's father's mother.