The term is a modern American football compound, formed within the English language. The element "third" follows the standard ordinal numbering system inherited from Proto-Germanic and ultimately Proto-Indo-European. The element "down" in this context has undergone a semantic shift from its original topographical meaning ("hill") to a specific sporting jargon, denoting a counted attempt or play. The compound "third-down" logically denotes the third attempt in the series, a critical juncture in the game's strategic progression, illustrating how specialized technical vocabularies are built from common native English elements.

Structure: third (ordinal number) + down (position/attempt) - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tritiyos. Functions as an ordinal numeral. - down: From Old English dūn (hill, mound), but in this compound, it derives from the sporting sense of "a phase of play" (as in "first down"), likely from the idea of being "down" on the field to attempt an advance.