Sourpuss is slang for a grouchy, bad tempered or sullen person.
A pouty child who is sullen because he did not get his own way is an example of a sourpuss.
Slang a person who has a gloomy or disagreeable expression or nature
Origin of sourpuss; from puss
A habitually gloomy or sullen person.
Origin of sourpusssour + puss2.