A person who is often gloomy or in a bad mood.
Being a sourpuss is not a good thing because it means the person is not happy and makes others feel unhappy too.
Examples:
She is always a sourpuss when she has to clean her room.
Don't be a sourpuss, let's have fun!
The sourpuss didn't want to play with us.
The cat looked like a sourpuss when we put the costume on it.
He's such a sourpuss, he never wants to do anything fun.
The old man was a real sourpuss, he never smiled at all.