Definition
go fuck oneself; fuck off.
Origins
Alteration of the biblical phrase 'be fruitful and multiply', a commandment to have many offspring.
How People Actually Use It
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."
Source: Urban Dictionary