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1.
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
 
2.
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