It means Definitely. Deffo is short hand for Definitely, just add a Jeffo for coolness!
Q 'You want to go to the movies?'
A 'Deffo Jeffo!'
by rachcook182 March 18, 2007
an longer version of definitely, usually to a person to person with a name like jefferson or geoff.
person 1.we should defo go to the the rave
person 2.yeah deffo jeffo
by work colleague April 01, 2011

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