lurker (n.)

c. 1300, lurken "to hide, lie hidden," probably from Scandinavian (compare dialectal Norwegian lurka "to sneak away," dialectal Swedish lurka "to be slow in one's work"), perhaps ultimately related to Middle English luren "to frown, lurk" (see lower (v.2)). From late 14c. as "move about secretly;" also "escape observation." Related: Lurked; lurking.