Old English sceadwe, sceaduwe "the effect of interception of sunlight, dark image cast by someone or something when interposed between an object and a source of light," oblique cases ("to the," "from the," "of the," "in the") of sceadu (see shade (n.)). Shadow is to shade (n.) as meadow is to mead (n.2). Similar formation in Old Saxon skado, Middle Dutch schaeduwe, Dutch schaduw, Old High German scato, German schatten, Gothic skadus "shadow, shade."
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.