If something is moving stagnantly, it is not moving at all. It is staying still and not going anywhere.
Stagnantly can make people think of things that are old, dirty, or not good.
Examples:
The water in the pond is moving stagnantly.
The air in the room was moving stagnantly.
The old pond water was moving stagnantly.
The old boat moved stagnantly through the water.
The old pond water was moving stagnantly, like it was dead.
The old pond water was moving stagnantly, like it was stuck in time.