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]

WordNet (r) 3.0 (2006):

slub
    n 1: soft lump or unevenness in a yarn; either an imperfection
         or created by design [syn: slub, knot, burl]