"Big back slang" refers to a style of speaking where words are reversed or altered to create a playful or coded language. It is often associated with specific cultural groups, particularly within certain communities in the UK. The phrase "big back slang" itself might imply a more elaborate or creative use of this type of language.
For example, in traditional back slang, the word "dog" would be said as "god." If you have a specific context or phrase in mind, I can provide more detailed information!