The word "tunnel" has a journey that traces back to the Old French "tonel," meaning a cask or a barrel, which itself derived from "tonne," a large cask. This connection likely arose from the visual similarity between a long, cylindrical barrel and a subterranean passage. The term was adopted into Middle English in the 15th century, initially referring to the nether part of a chimney or a flue, before its meaning specialized to its modern sense of an underground passage by the late 18th century. The semantic evolution beautifully illustrates how a concrete object (a cask) provided a metaphorical shape for an architectural and engineering feature.

The word is a simplex form. It entered English as a single lexical unit from Old French.