The Collaborative International Dictionary of English v.0.48:

Slubbing \Slub"bing\,
   a. & n. from Slub.
   [1913 Webster]

   Slubbing billy, or Slubbing machine, the machine by which
      slubs are formed.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slub \Slub\, n. [Etymol. uncertain.]
   A roll of wool slightly twisted; a rove; -- called also
   slubbing.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slub \Slub\, v. t. [imp. & p. p. Slubbed; p. pr. & vb. n.
   Slubbing.]
   To draw out and twist slightly; -- said of slivers of wool.
   [1913 Webster]