1540s, "displeasing to the eye," from un- (1) "not" + sight (n.) + -ly (1). Similar formation in 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;" unsightly is attested in Middle English only as an adverb meaning "invisibly" (late 15c.). Sightly as "pleasing to the eye" is from 1560s. Middle English also had unsighty "difficult or displeasing to look at" (early 15c., from sighty "attractive," late 14c.), also unsightily in the same sense (c. 1400).