Sulky, a. [Compar. Sulkier; superl. Sulkiest.]
Etymology: [See Sulkiness, and cf. Sulky, n.]
Moodly silent; sullen; sour; obstinate; morose; splenetic.
Syn. -- See Sullen.
1 other Webster entry under this spelling
Sulky, n.; pl. Sulkies.
A light two-wheeled carriage for a single person.
Note: Sulky is used adjectively in the names of several agricultural machines drawn by horses to denote that the machine is provided with wheels and a seat for the driver; as, sulky plow; sulky harrow; sulky rake, etc.
Shades of meaning
Fernald has no entry of its own for sulky; he lists it under morose.
Where he writes about it:
The sullen and sulky are discontented and resentful in regard to that against which they are too proud to protest, or consider all protest vain; sullen denotes more of pride, sulky more of resentful obstinacy. — from morose
The sullen and sulky are for the most part silent; the morose growl out bitter speeches. — from morose
Sullen and sulky moods may be transitory; one who is morose or surly is commonly so by disposition or habit. — from morose