Septic is connected to dangerous waste products. If your house is not connected to your town or city's sewer, your waste from drains and toilets goes into a septic tank.
Derived from a Greek word meaning "rotten," septic can indicate that something is infected. When a hospital patient goes into "septic shock," it means that their body, wracked by an infection, is starting to fail. People in septic shock are victims of a condition called sepsis.