four-fifths
UK[ˌfɔːˈfɪfθs]US[ˌfɔrˈfɪfθs]
n
A fraction representing four out of five equal parts of something; 4/5.
Morpheme Breakdown
four
fifths
four
number
fifths
ordinal number/plural noun
Etymology
The word "four-fifths" is a transparent compound formed from two native English number words. "Four" descends from the ancient Proto-Indo-European root for the numeral, which is remarkably stable across Indo-European languages. "Fifth" originates from the same PIE root for "five" (penkʷe), with the ordinal suffix '-th' added to denote position or division. The logic of the construction is purely arithmetic: it combines a cardinal numerator ("four") with a pluralized ordinal denominator ("fifths") to name a specific fraction, a pattern common in English for fractions above one (e.g., two-thirds, seven-eighths). This compound directly reflects its literal mathematical meaning without metaphorical evolution.
Analysis
Structure: four (number) + fifths (ordinal number/plural noun)
- four: From Old English fēower, from Proto-Germanic fedwōr, from Proto-Indo-European kʷetwóres. Functions as the cardinal number in the compound.
- fifths: From Old English fīfta (fifth), from Proto-Germanic fimftô, from PIE penkʷtos. The suffix '-s' indicates the plural form of the ordinal number, creating the fractional noun.
Examples
Approximately four-fifths of the Earth's atmosphere is nitrogen.
The new tax rate applies to the top four-fifths of income earners.
She had already completed four-fifths of the marathon when the rain started.