1. An act that expresses complete retardation. Derived from the condition "Trisomy 21" (Down's Syndrome)
2. Complete idiocy (see retarded)
Person A: Hey, did you see what Fred did last week?
Person B: Yeah, that was some Trisomy right there.
Person A: I'm a sneaky butterfly that rides on colorful unicorns, yay!
Person B: Trisomy, much?