five o'clock shadow
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?"
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Source: A Dictionary of American Idioms