month-old

The compound adjective "month-old" follows a straightforward and productive pattern in English for specifying age or duration. The first element, "month," traces its lineage to the Proto-Germanic mēnōth-, itself derived from the Proto-Indo-European root mē- meaning "to measure," with the lunar cycle serving as the original time-measuring unit. The second element, "old," originates from the Proto-Germanic aldaz, meaning "grown up, adult," from the PIE root al- meaning "to grow, nourish." The logic of the compound is transparently literal: it directly combines the measured time period ("month") with the state of having existed ("old") to denote something that has existed for precisely that period, a pattern analogous to "year-old" or "day-old."

Structure: month (time unit) + old (having a specified age) - month: From Old English mōnaþ, related to mōna (moon), indicating a unit of time based on the lunar cycle. Its functional role is as a noun specifying the unit of measurement. - old: From Old English eald, meaning "aged, ancient, of long existence." In this compound, its functional role is as an adjective-forming element meaning "having the age of."