Sullen

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Middle English (in the senses ‘solitary, averse to company’, and ‘unusual’): from Anglo-Norman French sulein, from sol ‘sole’.

wiktionary

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From Middle English solein, from Anglo-Norman soleyn(“alone”), from Old French sole(“single, sole, alone”), from Latin sōlus(“by oneself alone”). The change in meaning from "single" to morose occurred in Middle English.

etymonline

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sullen (adj.)

1570s, alteration of Middle English soleyn "unique, singular," from Anglo-French *solein, formed on the pattern of Old French solain "lonely," from soul "single," from Latin solus "by oneself, alone" (see sole (adj.)). The sense shift in Middle English from "solitary" to "morose" (i.e. "remaining alone through ill-humor") occurred late 14c. Related: Sullenly; sullenness.