The compound adjective 'year-long' is a transparent formation from two native English elements. 'Year' originates from the Proto-Germanic 'jērą', related to the cycle of seasons, while 'long' stems from Proto-Germanic 'langaz', denoting extended duration. Their combination in Modern English follows a straightforward compounding pattern common in Germanic languages, where a noun modifies an adjective to create a descriptive term. The literal meaning, "having the duration of a year," directly reflects the sum of its parts, illustrating a logical and enduring method for constructing temporal descriptors.

Structure: year (time period) + long (duration) - year: From Old English 'gear', meaning a cycle of seasons or a period of twelve months. It functions as a noun specifying the unit of time. - long: From Old English 'lang', meaning having great duration or extent. It functions as an adjective describing the temporal quality.