The Collaborative International Dictionary of English v.0.48:

Scarf \Scarf\, n.
      (a) In a piece which is to be united to another by a scarf
          joint, the part of the end or edge that is tapered
          off, rabbeted, or notched so as to be thinner than the
          rest of the piece.
      (b) A scarf joint.
          [1913 Webster]

   Scarf joint
      (a) A joint made by overlapping and bolting or locking
          together the ends of two pieces of timber that are
          halved, notched, or cut away so that they will fit
          each other and form a lengthened beam of the same size
          at the junction as elsewhere.
      (b) A joint formed by welding, riveting, or brazing
          together the overlapping scarfed ends, or edges, of
          metal rods, sheets, etc.

   Scarf weld. See under Weld.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scarf joint
    n 1: a joint made by notching the ends of two pieces of timber
         or metal so that they will lock together end-to-end [syn:
         scarf joint, scarf]