When a group of government agents (i.e. FBI, DEA, ATF, etc.) are enacting a search warrant on a premises. So named because of the large amount of agents that wear trenchcoats as part of their attire. Term used more often in colder regions of the country. Can also apply to municipal law enforcement detectives as well.
A loosely aligned group of misfits at Columbine High School in the time of the Columbine Shootings. Eric Harris and Dylan Klebold were members of the Threnchcoat Mafia. Harris and Klebold had advocated 'killing jocks' but ended up doing something wrong instead. 'Trenchcoat Mafia' now indicates any similar group of losers with a patina of simmering menace.
My former academic excellence went into a tailspin in my senior year. I lapsed from all good society and fell in with the school's trenchcoat mafia. We cut class en masse and hung out in the library learning boot code and posing as text lesbians in chat rooms.
A clichéd character in a story, movie, anime, or whatever who wears a black trenchcoat and has long hair of any color. Trenchcoaters always are mysterious loners and have no pasts. Their names comprise of either some "badass" name like Deathbringer, some gibberish, Japanese sounding name like Kajimaru (usually there is "maru" at the end), or both. They almost always carry katanas and have mastered them all through their 15-17 years of living and often carry machineguns or other heavy weaponry. Usually they are very uninteresting and stupid.
12 year old kid: The main character of my story "CRIMSON TIDES OF DEATH!!1" is a mysterious loner with no past named Vincent who carries a katana, a nuke, and a machinegun! IM GONNA TRY AND GET TIHS PUBLISHD
Other person: Your character is a trenchcoater. I can already tell your story's gonna suck.