"Back of the *** packet" is apparently a British equivalent to the American "back-of-the-envelope" which is an adjective almost universally applied to highly informal writing or calculation, often done in haste, not intended for wide use.
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A F.a.g. Packet is a cigarette carton. f.a.g. (minus the full stop) is a slang word for a cigarette in the UK. hence, when an engineer needed to show a colleague a quick sketch, he would often draw it on the back of his cigarette carton, or f.a.g. packet.
The definition is very similar to the one already put forward, as in a rushed calculation, not meant to be taken as absolutely accurate or to be fully relied upon.
But there is no need for the full stops between the letters in the word "***", as certainly in the UK, the word *** simply means a cigarette and nothing more.
I know it has other conatations in other countries, especial