Green’s Dictionary of Slang

a street preacher.

1799		Historical, Geographical, Commercial, and Philosophical View I 385: The English word that conveys the proper meaning of Tunkers is Sops or Dippers .
1858		Elgin Courier 8 Jan. 2/5: A Free tunker has forgotten to send in his name.
1867		Dixon New Amer. II 184: The Tunkers [...] profess Baptist tenets; and the word ‘tunker’ meaning to dip a crumb into gravy, a sop into wine, they are described by those who use it, in a very poor joke, as dippers and sops. [...] We English style them Dunkers, by mistake .
1906		Aberdeen Jrnl 13 Mar. 7/4: A Dipper is a member of the Baptist body, and so also is a ‘Tunker,’ [...] the word being derived from the German ‘tunken’ or ‘dunken,’ to dip.