Etymology
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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.

updated on December 28, 2013

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Definitions of sullen from WordNet

sullen (adj.)
darkened by clouds;
Synonyms: heavy / lowering / threatening
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.