The compound adjective "thirty-year" is a modern English formation combining two ancient Germanic elements. "Thirty" descends from Old English þrītiġ, a derivative of "three," indicating three tens. "Year" originates from the Old English ġēar, a term for the annual seasonal cycle, traceable to Proto-Indo-European yeh₁r- meaning "year" or "season." The compound follows the common English pattern of number-noun modifiers (e.g., five-day, ten-mile) to create a single attributive adjective describing duration or applicability. Its logic is transparently literal, directly specifying a timespan of three decades, and is frequently used in financial, legal, and demographic contexts (e.g., thirty-year mortgage, thirty-year trend).

Structure: thirty (number) + year (time period) - thirty: From Old English þrītiġ, related to þrīe (three). A numeral indicating the quantity 30. - year: From Old English ġēar, from Proto-Germanic jērą, meaning a period of twelve months or the cycle of seasons.