Sulky means silent and cross because you did not get your way. The mood that will not say what is wrong.
The silence is the essential part, which is what separates sulking from complaining, since a sulk is a protest made by withholding. It belongs to children and to adults behaving like them. The verb is sulk and the noun is a sulk, so somebody can go off in a sulk. There is a completely unrelated noun, since a sulky is a light two-wheeled cart used in harness racing.
1adjectivesulkySilently bad-tempered because of a grievance.
“He was sulky for the rest of the afternoon.”
See sulk. The vehicle is so called as it obliges the rider to be alone, as though sulking.
Etymology from Wiktionary, CC BY-SA