The act of slowly killing yourself by the use of tobacco or drugs. This happens over the years as dangerous substances are put into your body. They slowly kill you, hence slow-motion suicide.
The prof: Does anyone know what someone is doing when they sit outside on their porch smoking cigarettes all the time?
Nick: Killing themselves?
Greg: No dumbass! shit! they are committing slow-motion suicide...jeez....stupid college kids these days
The Prof (thinking to himself): Im failing him, and him, and maybe her, probably him....