語源はPIE*penkweの5であり、五角形、五重奏と同じである。
[under picture of a pretty girl] "If I were a man I'd pay attention to that phrase '5 O'Clock Shadow.' It's that messy beard growth which appears prematurely about 5 P.M." [Advertisement for Gem razors and blades, Life," May 9, 1938]