The word wraith, marked by the Oxford English Dictionary as being of obscure origin, is first attested in 1513, with the meaning of ghost or spectre (that is, an apparition of a living or once-living being, possibly as a portent of death). In 18th century Scotland it was applied to water spirits, and in England it became used in a metaphoric sense to refer to wraith-like things, and to portents in general.
In the case of a living person being doubled it is thought that the wraith is an omen of the person's imminent death.
The word may be of Scots origin, possibly through Old Norse vörðr, meaning guardian, and related to Irish arrach, meaning apparition. An association with the verb writhe has also been claimed; Tolkien held this view [1], and his use of the word in the naming of the creatures ringwraiths has influenced writers of fantasy literature, television shows, and games, who use it with its meaning of a shadow-thing, a spirit of another world.