The compound adjective "three-week" is a modern English formation built from two ancient, foundational elements. The numeral "three" descends from the Proto-Indo-European root tréyes, a concept so essential it is reflected in cognates across numerous Indo-European languages. The noun "week" originates from a Germanic root related to the concept of "turning" or "succession," reflecting the cyclical nature of time. When combined, these morphemes follow a straightforward, logical compounding pattern in English, where a number directly modifies a unit of time to create a precise adjectival descriptor for duration. The evolution is not of semantic shift but of syntactic combination, merging ancient lexical items to fulfill a modern descriptive need.

Structure: three (number) + week (time period) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a cardinal number. - week: From Old English wice, wicu ("a turning, succession, change"), from Proto-Germanic wikōn ("turn, succession"), likely from Proto-Indo-European weik- ("to bend, wind"). Functions as a noun denoting a period of seven days.