thunder +‎ box; in the sense of a toilet, presumed to be because of the noises that may be made while using it, especially while defecating.

What is thunderbox slang for?

/ (ˈθʌndəˌbɒks) / noun slang. a portable boxlike lavatory seat that can be placed over a hole in the ground. any portable lavatory.

What does thunderbox mean in Australia?

Filters. (historical) A close-stool, a stool enclosing a chamber pot. noun. (UK, Australia, slang) A rudimentary outside toilet.