The word "tum" is a classic example of hypocorism, the formation of pet names or diminutives through abbreviation and playful alteration. It emerged in the 1800s as a childish or affectionate shortening of the word "stomach," likely influenced by the natural tendency in baby talk to simplify consonant clusters. The original "stomach" derives from Latin stomachus and Greek stomachos, which referred to the gullet or throat as well as the organ of digestion, linked to stoma meaning "mouth." Thus, "tum" represents the final stage of a long etymological journey, collapsing a classical term for an opening or orifice into a simple, monosyllabic nursery word for the belly.

This is a simplex word. It is a clipped, informal alteration of the word "stomach," originating in British English in the mid-19th century. Its functional role is as a colloquial noun.