"Brown bread"? It's a New England tradition in the US. I baked some a few times, many years ago. I've never seen it used as a euphemism for dead, defunct, bought the farm, an ex-parrot, etc. Is that from some language other than US English? It doesn't register with my (limited) knowledge of the Queen's English, either.
Its Cockney Rhyming Slang - brown bread means dead
Essence of Cockney Rhyming Slang is you have two words in a phrase where second rhymes with word. In fact, you often drop the second word e.g.
in a pub, you may order a Gold (Gold Watch - Scotch)
The phrase "Let's have a butcher's" means a look (Butcher's Hook - Look)
A girl may have a nice Boat (Boat Race - Face).
Sometimes the phrase has 3 words like Horse and Trap but you ignore the "and".
I leave it up to you to work out what I mean when I say I am going for a Brace.
Clue: Brace and .......
Cockney Rhyming Slang was invented by Londoners working in the docks as a way of having private conversations as they used to get annoyed with the imported Irish Labourers slipping into Gaelic when they did not want Londoners to understand.
It is surprising how many Cockney Rhyming Slang terms are used in UK.
As I am in a pub, I am off for a Jimmy (Riddle), going up the Apple (and Pears), then I am going to have a final drink for the Frog (and Toad). I will probably have a Thelma (Ritter). Then I will call a taxi on the Dog (and Bone)