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early 15c., "displeasing to the eye," from un- (1) "not" + sight + -ly (1). Cf. Middle Dutch onsichtlijc "invisible; ugly," Middle High German unsihtlih "invisible." Related: Unsightliness. Middle English sightlie is attested from mid-15c. but only in the sense "visible;" sightly as "pleasing to the eye" is from 1560s.