once-in-a-generation

The phrase is a modern English compound adjective, constructed in the 20th century by combining fundamental, native English grammatical words ('once', 'in', 'a') with the noun 'generation'. The noun 'generation' itself has a rich history, entering Middle English from Old French generacion, which derived from Latin generationem (nominative generatio), meaning "a begetting, procreation." This Latin term stems from the verb generare, "to bring forth, produce," which is based on genus (birth, race, kind), a root tracing back to the Proto-Indo-European ǵénh₁os (race, lineage). The logic of the modern phrase is entirely transparent and metaphorical: it takes the concrete concept of a human lifespan cohort (a generation) and uses it as a standard measure of time to hyperbolically express unparalleled rarity or epochal importance, implying such an event is not expected to recur for roughly 25-30 years.

This is a hyphenated compound phrase, not a single word formed from classical morphemes. It is built from common English words: 'once' (adverb), 'in' (preposition), 'a' (article), and 'generation' (noun). Therefore, a meaningful morphemic segmentation into etymological roots is not applicable.