A poor excuse for a mustache, generally sported by: military personnel, awkward loser teenagers, trailer trash, rednecks, Mexicans/Latinos, and/or police officers. A trashstache is typically worn by men that lack the balls or coolness to wear a characterful mustache, i.e. handle bar, 70's pornstache, freak-show carnival announcer stache, Hitlerstache, 18th century safaristache.
I saw a poser gothic teenager at the mall today. He was wearing an Insane Clown Posse t-shirt, baggy hot topic pants, had a crew cut, and trashstache.
Military men trying not to look stupid with a high and tight or trying to blend in with society grow trashstaches.