shadow (n.)youdaoicibaDictYouDict[shadow 词源字典]
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 (Ps. xxiii:4, etc.), which itself translates Greek skia thanatou, perhaps a mistranslation of a Hebrew word for "intense darkness." In "Beowulf," Gendel 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.[shadow etymology, shadow origin, 英语词源]
shadow (v.)youdaoicibaDictYouDict
Middle English schadowen, Kentish ssedwi, from late Old English sceadwian "to protect as with covering wings" (also see overshadow), from the root of shadow (n.). Similar formation in Old Saxon skadoian, Dutch schaduwen, Old High German scatewen, German (über)schatten. From mid-14c. as "provide shade;" late 14c. as "cast a shadow over" (literal and figurative), from early 15c. as "darken" (in illustration, etc.). Meaning "to follow like a shadow" is from c. 1600 in an isolated instance; not attested again until 1872. Related: Shadowed; shadowing.