ˌfive o'clock ˈshadow noun [singular] the dark colour on a man’s chin where the hair has grown during the day
[TahlilGaran] Dictionary of Contemporary English ▲
five o'clock shadow n. phr. A very short growth of beard on a man's face who did shave in the morning but whose beard is so strong that it is again visible in the afternoon.
"You have a five o'clock shadow, honey," Irene said, "and we're going to the opera. Why don't you shave again quickly?" [TahlilGaran] English Idioms Dictionary ▲