Green’s Dictionary of Slang

1. a religious or political orator.

1607		Middleton Widdow of Watling-streete I iv: nich.: Our parson rails against players mightily, I can tell you, because they brought him drunk upo’th’ stage once. [...] corp.: I cannot blame him then, poor church spout.
1885		‘’Arry on the Elections’ in Punch 12 Dec. 277/2: I pelted the Radical posters, I guyed all the Radical spouts.

2. speechifying, haranguing.

1855		Broadway Belle (NY) 29 Jan. n.p.: The Spout Shops. Otherwise called the theatres.
1866		Sportsman 18 Dec. 2/1: Notes on News [...] He commenced with long spout about the French Revolution, equality, fraternity, rights of woman, theology, and so on.
1878		‘’Arry at the Play’ in Punch 2 Nov. in P. Marks (2006) 40: What I ’old is plays should be plays, and not hist’ry, or preachin’, or spout.
1884		Sporting Gaz. (London) 11 Oct. 1286/1: Then a ‘reverend’ moved two resolutions which, in Western slang would he described as ‘ whole hog spouts’.

3. a large and ever-open mouth.

1909

J. Ware Passing Eng. of the Victorian Era.