means A passage dug horizontally through earth, rock, or beneath water, letting roads, trains, or people pass straight through what would otherwise block the way.

from From Middle English 'tonel', borrowed from Old French 'tonnel' — a cask or barrel, a diminutive of 'tonne' (tun). The word started life meaning a rounded, barrel-shaped vessel or net, and the sense slid toward 'a tube-shaped passage' as English speakers seized on the hollow, curving shape. So beneath every mountain-piercing tunnel lies, etymologically, a humble little wine barrel.