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1.
Originally a bible phrase, Brits now use it when they mean 'fuck off'.

A polite way of saying you told someone where to go.
I told the door-to-door salesman to go forth and multiply.
by mj5 January 10, 2008
 
2.
Basically it means to fuck off. Use this phrase as a euphemism when you don't want to be obscene in telling someone that their presence is not desirable.
When the boss of Taco Bell went to his workers and asked them to take a pay cut, they all told him to go forth and multiply.
by Iconoklastic~ March 22, 2010
 
3.
How to tell a mixed-gender group of your employees that you said to someone "Go fuck yourself."
I told that gimuntuan grandmother fucker and ass-covering tea bag wearer FT to go forth and multiply.
by Phak Yoo November 17, 2004