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.