The word "subway" is a transparent compound formed within English from two native elements. The prefix "sub-," borrowed from Latin sub meaning "under," has been a productive component in English since the Middle English period. It was combined with the Old English noun "way," which traces back to the Proto-Germanic wegaz and ultimately to the Proto-Indo-European root weǵh- meaning "to go, transport." The literal meaning, "an underground way or path," perfectly captures the modern definitions. It first described an underground pedestrian passage in the 1820s, with its application to underground urban railways emerging in the late 19th century, most famously with the London Underground and the New York City Subway systems.

Structure: sub (under) + way (path) - sub: A prefix of Latin origin, meaning "under, below, or beneath." It functions as a locative modifier. - way: A noun of Old English origin (from weg), meaning "a path, road, or course for traveling." It functions as the core root.