Five o'clock shadow
Beard regrowth that darkens a man's features late in the day, following a morning shave.
In the 1980s, designer stubble became fashionable, based largely on the popularity and 'coolness' of stars like George Michael, Don Johnson and, before them, Clint Eastwood's 'Man with no name' character. This was quite an achievement as, prior to then, a swarthy and unshaven appearance was considered to be reserved for ruffians and ne'er do wells. Respectable men were expected to be either clean-shaven or to have a full moustache or beard. If a man planned to grow a beard, he usually waited until he was away on a holiday and not seen in public for a few days, until the 'five o'clock shadow', as it was then universally called, phase was passed.
Why 'five of the clock'? Why not four or six? The 'five o'clock shadow' coinage was based on the 19th century upper-crust English habit of taking tea at five o'clock. Not that the notably upper-crust 7th Earl of Shaftesbury had much time for it. He is reported in Edwin Hodder's biography, 1886, as saying:
Five o'clock tea, that pernicious, unprincipled and stomach-ruining habit.
Nevertheless, the teas became popular with the middle-classes and became known as 'five o'clocks' and, when the habit travelled the Atlantic to the USA, light late-afternoon meals were renamed 'five o'clock dinners'.
Step forward to the 1930s and into the marketing department of the Gem Safety Razor Company. While dreaming up a new advertising campaign, they decided to try and convince previously unsuspecting men that they suffered from 'ugly, afternoon beard growth' and that this could only be countered by the purchase and use of 'Gem Micromatic Blades'. Needing a snappy name for this late-afternoon ailment, which would of course bar sufferers from any genteel 'five o'clock dinner', they chose to call it 'five o'clock shadow'.
See other phrases that were coined in the USA.