The Collaborative International Dictionary of English v.0.48:

Excrescent \Ex*cres"cent\, a. [L. excresens, -entis, p. pr. of
   excrescere to grow out; ex out + crescere to grow. See
   Crescent.]
   Growing out in an abnormal or morbid manner or as a
   superfluity.
   [1913 Webster]

         Expunge the whole, or lip the excrescent parts. --Pope.
   [1913 Webster]

   Excrescent letter (Philol.), a letter which has been added
      to a root; as, the d in alder (AS. alr) is an excrescent
      letter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

excrescent
    adj 1: forming an outgrowth (usually an excessive outgrowth)