The Collaborative International Dictionary of English v.0.48:

Jeerer \Jeer"er\, n.
   A scoffer; a railer; a mocker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

jeerer
    n 1: someone who jeers or mocks or treats something with
         contempt or calls out in derision [syn: scoffer,
         flouter, mocker, jeerer]