Buttoxin
When a person releases a particulary nasty smelling fart into the room.

a fart which is released from the buttocks, which could be described as being toxic.

hence the name buttoxin.

Butt + toxin/toxic = buttoxin.
Jim: *loud fart*

Jenny: "eugh, that smells disgusting!"

Jack: "that's toxic man."

Jim: "that's a true buttoxin!"

Jenny: "so nincompadrae!!"