The compound "third-row" is a modern English formation that transparently combines two native English elements. "Third" descends from the Proto-Indo-European numeral system, specifically the ordinal form derived from trei- (three), which evolved through Germanic into Old English þridda. "Row" originates from an Old English word for a line or series, likely connected to the concept of riding in a line. When combined, the literal meaning "third line" logically evolved to specify a position, most concretely applied to the third line of seats in a vehicle like a car or minivan, reflecting a straightforward descriptive naming convention.

Structure: third (ordinal number) + row (line) - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tri-ti- (third). Functions as a numerical determiner. - row: From Old English rāw (a line, series), from Proto-Germanic raigwō, related to rīdan (to ride). Functions as a noun denoting a linear arrangement.