A nightmare trail is a mass of usually thick, dark, and over-exposed hair on a person's lower back which leads down their butt crack. A nightmare trail is similar to a happy trail except that it is not happy.
Did you see Shanon's nightmare trail at the beach last weekend?
A skill on a League of Legends character, Singed, The Mad Chemist involving the act of leaving a cloud of chemicals that poison the enemies behind you as you walk.
In real life, the act of knowingly walking in front of people and silently farting as you stroll.
While this may seem douchebaggy, it is quite fun and thrilling to do, especially if you have headphones on so you are unaware of the volume of your exhaust fumes.
As I walked out of McDonald's yesterday, I left a silent, yet deadly poison trail from the counters to the playground.