once-in-a-lifetime

The phrase "once-in-a-lifetime" is a modern English lexicalized compound, emerging in the 20th century to describe uniquely rare events. Its logic is transparently literal, built from core Germanic and Old English vocabulary: "once" (from Old English ānes, meaning 'one time'), "in" (from Old English in), and "lifetime" (from Old English līf 'life' + tīma 'time'). There is no classical root; its formation follows the English pattern of creating adjectival phrases through hyphenation. The entire construction serves as a single conceptual unit, emphasizing supreme rarity by framing an event as occurring only once within the entire span of a human life.

This is a hyphenated compound phrase, not a single morpheme-based word. It is formed from common English words: 'once' (adverb), 'in' (preposition), 'a' (article), and 'lifetime' (compound noun: 'life' + 'time').