The Collaborative International Dictionary of English v.0.48:

Rapper \Rap"per\ (r[a^]p"p[~e]r), n. [From Rap.]
   1. One who, or that which, raps or knocks; specifically, the
      knocker of a door. --Sterne.
      [1913 Webster]

   2. A forcible oath or lie. [Slang] --Bp. Parker.
      [1913 Webster]

   3. A musician specializing in rap music.
      [PJC]

WordNet (r) 3.0 (2006):

rapper
    n 1: someone who performs rap music
    2: a device (usually metal and ornamental) attached by a hinge
       to a door [syn: knocker, doorknocker, rapper]