From mid-13c. as "darkened area created by shadows, shade." From early 13c. in sense "anything unreal;" mid-14c. as "a ghost;" late 14c. as "a foreshadowing, prefiguration." Meaning "imitation, copy" is from 1690s. Sense of "the faintest trace" is from 1580s; that of "a spy who follows" is from 1859.
As a designation of members of an opposition party chosen as counterparts of the government in power, it is recorded from 1906. Shadow of Death (c. 1200) translates Vulgate umbra mortis (Psalms xxiii.4, etc.), which itself translates Greek skia thanatou, perhaps a mistranslation of a Hebrew word for "intense darkness." In "Beowulf," Grendel is a sceadugenga, a shadow-goer, and another word for "darkness" is sceaduhelm. To be afraid of one's (own) shadow "be very timorous" is from 1580s.
Old English ofersceadwian "to cast a shadow over, obscure;" see over + shadow (v.). It was used to render Latin obumbrare in New Testament, as were Middle High German überschatewen, Middle Dutch overschaduwen, Gothic ufarskadwjan in those languages. Figurative sense is from 1580s. Related: Overshadowed; overshadowing.