means The process by which something — liquid, an effect, a crowd, an emotion — extends beyond its intended container or boundary into an adjacent space.

from A transparent English compound of "spill" + "over," built on the same logic you can watch happen at a dinner table when a glass tips. "Spill" descends from Old English "spillan," meaning to destroy, kill, or waste — only later softening into the gentler sense of letting liquid escape. "Over" is the ancient Germanic word for above and beyond. The literal noun is old, but the figurative "spillover" — effects bleeding into other domains, economies, ecosystems, conflicts — is a 20th-century habit of speech, fond of treating consequences like liquid that won't stay in its cup.