What is the etymology behind (word history of) the English word,
"Uncertain origin" is not a good answer
...but it will have to do*
(*But at least, now we know that "wraith" is Scottish ,
and either Klein or Weekley may be right )
*Until we have more information...
edit: I replied to a post which afterwards was rewritten (poor etiquette! ).
Why not both? Since both Old Norse and Erse (Scots Gaelic) were spoken in Scotland, people could have heard both words used, in somewhat similar contexts, and "folk-conflated" them.
Separate names with a comma.