late 12c., "power of discernment," from Old Norse skil "distinction, ability to make out, discernment, adjustment," related to skilja (v.) "to separate; discern, understand," from Proto-Germanic *skaljo- "divide, separate" (cognates: Swedish skäl "reason," Danish skjel "a separation, boundary, limit," Middle Low German schillen "to differ," Middle Low German, Middle Dutch schele "separation, discrimination;" see shell, n.). Sense of "ability, cleverness" first recorded early 13c.