A mustachioed Samuel Kirk
A mustache, or in slang 'stache, was a form of facial hair grown between the upper lip and nose. It was a component of a beard.
The mustache was a common evil trait in Human storytelling. In the wake of Admiral Norah Satie's witch-hunts in 2367, Captain Jean-Luc Picard told Lieutenant Worf, who was disgusted with his own conduct (he had strongly supported, and even assisted, her investigations until she made an accusation against Picard which he knew to be false), that "villains who twirl their mustaches are easy to spot. Those who clothe themselves in good deeds are well camouflaged." (TNG: "The Drumhead")
In 2369, Q sported a mustache while sparring with Benjamin Sisko. (DS9: "Q-Less")
When Andy Billups was confronted by his mother, Queen Paolana, in 2380, she took note of his appearance, observing that he was "wasting away", and then asked "[..] what happened to that mustache? It is thinning." Later, during the aborted Royal Copulation, one of the Queen's royal guards commented on how Billups' "[..] 'stache is so regal." (LD: "Where Pleasant Fountains Lie")