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You can play along with The Longest Johns video in GCEA tuning, if you put your capo on the 3rd fret.
The Weller Bros. was a Sydney Australia shore-whaling company that operated primarily along the southern coast of New Zealand from 1830 to 1840. Though they had numerous vessels in their employ, none appear to have been named the Billy o' Tea. Anyone under the employ of the Weller Bros. were referred to as a Wellerman, but in the specific context of the song the Wellerman takes on the meaning of a supply ship sent by the company.
A "billy" is slang for a makeshift kettle used for boiling water to make tea.
Tonguing refers to the tonguers, the part of a shore whaling team responsible for butchering the whale after it had been caught.