Cockney rhyming slang for internet shorthand 'tl;dr' meaning 'too long; didn't read'.
Used to tell someone to just summarize what they're saying, because you can't be arsed to to go through it all. Also used to denote such a summary.
Most notably, 'blue fawn' provides a means to convey 'tl;dr' in face-to-face conversation for comedic or ironic value.
As is common in Cockney rhyming slang, the slang term is not necessarily related to, or rhyming with, the original word, as it may represent multiple iterations of rhyme, synonym, or ironic interpretation.
In this case, 'blue fawn' is a synonym for 'teal deer' a possible pronunciation of 'tl;dr'
A: <unnecessarily long, overly verbose, meandering explanation of something with no point or end in sight>
B: Fucking 'ell man. Just give me the blue fawn of the matter will ya or we'll be 'ere all night!