Green’s Dictionary of Slang

(orig. Irish) definitely.

1966		T. Keyes All Night Stand 68: Even if you were on for deffo, free and no trouble.
1977		A. Bleasdale Who’s Been Sleeping in my Bed 217: ‘Deffo, Snotty,’ I said.
1986		A. Bleasdale No Surrender 24: Oh, deffo.
1994		(con. 1978) G. Byrne Pictures in my Head 89: He turns me this way and that. ‘Right side, deffo.’.
1998		I. Welsh Filth 244: He’s defo lost the plot since that move to Traffic.
2006		G. Malkani Londonstani (2007) 93: If it don’t happen before the wedding, it’ll deffo happen on the day.
2009		J. Joso Soothing Music for Stray Cats 90: Some Richard Ashcroft too, defo [...] boss stuff.
2011		J.J. Connolly Viva La Madness 354: ‘Where’re they firing from?’ ‘It’s the red-brick building, deffo, up to the right’.
2012		(con. 1980s) I. Welsh Skagboys 343: Ali saw um earlier that night, sais he wis defo oan the warpath.
2016		Times Times2 16 Dec. 🌐 CW thinks Jesus could deffo headline above Bieber .
2018		G. Gilmore Base Nature [ebook] ‘Potential client who I deffo want to talk about first’.
2020		Twitter 16 Mar. 🌐 Hey pals, sorry for the silence, but I’m deffo turning a corner!
2021		Central Cee ‘Pinging’ 🎵 Got my cake up, I paid it straight up, I defo don't need co-signing (Nah).