Wiktionary
deff
adv. (context Internet slang English) (alternative form of definitely English)
Usage examples of "deff".
Walking beside Deffer, the dandified one, is Chaka, Baglio's accountant and trouble-shooter.
If Henry Deffer had been alerted by their muffled voices in Keesey's room just below his own, he was playing it very cool indeed.
That was all Tucker had to see to understand why Baglio, a much younger man, was in the driver's seat figuratively, while Deffer was there literally.