The fandom's nickname for the moustache of Dr. John Watson, which appeared briefly at the beginning of BBC Sherlock's third series. During 'The Empty Hearse' several jokes are made about the Johnstache, until eventually John shaves it off, famously saying "I don't shave for Sherlock Holmes!"
When teenage boys begin to grow their first facial hairs and refuse to shave them, thus growing nothing but a pathetically small mustache/ some chin hairs.
When a person, usually a guy, but girls have them too, hasn't shaved their face and it looks like they have dirt above their lip. Sometimes girls bleech theirs. Mexican children usually have them. James has one. Thats where the name originated.
I'm glad Adam shaved his Jamestache before banquet. He looks better. I wish Jen would realize hers and bleach it.