Sulk

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late 18th century: perhaps a back-formation from sulky.

wiktionary

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Back-formation from  sulky, of uncertain origin. Probably from Middle English *sulke,  *solke (attested in  solcennesse(“idleness; laziness”), from Old English āsolcennys(“idleness; slothfulness; sluggishness; laziness”), from  āsolcen(“sulky, languid”), from past participle of Old English āseolcan(“be slow; be weak or slothful; languish”), from Proto-Germanic *selkaną(“to fall in drops; dribble; droop”), from Proto-Indo-European*sélǵ-o-nom, from  *selǵ-(“to let go, send”). Cognate with several Indo-Iranian words deriving from Proto-Indo-Iranian *sarȷ́- [1] (such as Sanskrit सृजति(sṛjáti),  सर्जति(sarjati),  सर्जन(sárjana),  सृक(sṛká)), possibly Hittite 𒊭𒀠𒀝𒍣(ša-al-ak-zi/šalkzi/, “knead, mix”), although the semantic connection is weak. [2]

Latin sulcus.

etymonline

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sulk (v.)

1781, back-formation of sulky (adj.). Related: Sulked; sulking. As a noun from 1792.