Green’s Dictionary of Slang

tub-thumper n.

also tub-drubber, -hunter, -pounder, -puncher
[tub n.1 (2) + SE thumper/drubber; ‘from the old Puritan fashion of “holding forth” from a tub, or beer barrel, as a mark of their contempt for decorated pulpits’ Hotten (1864)]

1. [18C+] a vehement preacher or orator, either clerical or secular.

2. (UK tramp) a cooper.