The word "spillover" is a transparent compound formed from two native English elements. The verb "spill" has a complex history, originating from an Old English term with violent connotations of destruction, which softened over centuries to denote a more mundane accidental flow. The particle "over," with its ancient roots in Proto-Germanic, consistently denotes spatial or conceptual excess. When combined in the early 20th century, their literal meanings fused perfectly: a liquid "spilling" and going "over" its boundary. This concrete image was then logically extended into economics and sociology in the mid-20th century to describe effects that escape their original confines, making the term a vivid metaphor for unintended consequences that cascade from one domain into another.

Structure: spill (to flow out) + over (above, beyond) - spill: From Old English spillan meaning "to destroy, kill, waste." Its modern sense of "to cause (liquid) to fall or run out" developed in Middle English. In this compound, it functions as the core verb root indicating an unintended flow or release. - over: From Old English ofer, a preposition/adverb of Proto-Germanic origin, meaning "above, across, beyond." In this compound, it functions as an adverbial particle specifying the direction or scope of the spill.