The word 'vat' is a direct descendant from the Old English fæt, a common Germanic term for a vessel or container. Its lineage can be traced back to the Proto-Germanic \fatą, which carried the same core meaning. This root is further connected to the Proto-Indo-European \pod-, meaning "vessel," a base also seen in the Latin pōtus ("a drink"), linking the concept of a container to its function of holding liquids. Over centuries, the word's spelling evolved from fæt to vat, and its meaning narrowed from a general term for any vessel to specifically denote a large tub or tank used in industrial or manufacturing contexts.

This is a simplex word. It is a loanword from Old English fæt, meaning "container, vessel," which is of Germanic origin and related to Dutch vat and German Fass.