Do you have an example sentence?
One possibility is, "Those girls are such dummies." This implies that they are dumb/stupid.
The dummies sat in wet paint.
That dummy couldn't figure out the answer.
Another could be a ventriloquist dummy, which is a type of doll performers use.
The man and his dummy had a hilarious act that had everyone laughing.
The creepy dummies lined the walls, their eyes following me around the room.