The Collaborative International Dictionary of English v.0.48:

Tunker \Tun"ker\ (t[u^][ng]"k[~e]r), n. (Eccl.)
   Same as Dunker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Tunker
    n 1: an adherent of Baptistic doctrines (who practice baptism by
         immersion) [syn: Dunker, Dunkard, Tunker]