Mephitic means something that is very bad-smelling or poisonous, like a gas.
Mephitic things are very dangerous and can make you sick or hurt you.
Examples:
The mephitic gas made everyone sick.
The mephitic smell was so bad, we had to leave the room.
The mephitic fumes filled the air.
The mephitic odor was so bad, it chased all the animals away.
The mephitic swamp was so stinky, no one could go near it.
The mephitic cloud hung over the town, making everyone feel sick.