"... to piss in someone's pocket ...", phr. [1940s+] (Aus.) - to curry favour, to be extremely close to someone, to ingratiate oneself.
If someone tells you to "Don't piss in my pocket" and intones it in jocular, familiar terms they are indicating that they reckon that you are taking a lend of them, trying to put one over them, trying to 'pull the wool over their eyes', or, even 'taking the Mickey out of them'.
But what if someone says it to you in an aggro format, with an indication of anger or affront? In this case . It means “don’t take me for a fool, don’t try to deceive me, don’t flatter me with your lies”.
In Australia it is common or vulgar language, often the domain of the working class who are in no way backwards in letting someone know where they stand and what they think of the other person and, frequently, they do this by use of colloquial phrases.
As they are not raised and educated in an insular fashion, most Australian schoolboys know the phrase, even if they are too genteel to use it in common use. In most practical ways, Australia is an egalitarian society. This does not mean that everyone is the same or that everybody has equal wealth or property. But it does mean that there are no formal or entrenched class distinctions in Australian society, as there are in some other countries.
Australians tend to be gregarious and outgoing. Most are relatively informal, socially and in their relationships with acquaintances and work colleagues, so much so that the use of such phrases comes easily to many tongues in everyday interactions..
Fair enough .... but .... what is the origin of this term?
Michael Quinion, writing in "World Wide Words" , 1996–2012, says:
"It’s a modern Australianism, recorded from the 1960s, but a precursor — pissing down any one’s back — is recorded in the same sense in the 1811 enlarged edition of Francis Grose’s Dictionary of the Vulgar Tongue."There is another version which states: "Don’t piss down my leg and tell me it’s raining" and has its origins from the earlier English expression of: Don’t piss down my back and tell me it’s raining!
The reason why it’s obscure is that its only half the expression. Don’t piss in my pocket is a shortened form of don’t piss in my pocket and tell me it’s raining. It means “don’t take me for a fool, don’t try to deceive me, don’t flatter me with your lies”.
So, If an Australian says to you "Don’t come the raw prawn with me - Stop pissing in my pocket!" they are telling you that they don't believe you and you better not try and treat them like a fool or suffer the consequences.
"Don't come the raw prawn with me?" Oh dear! Another can of worms that will have to wait for another day!