It's true this word is in some dictionaries, sometimes as
deffo. But as you say, anona, it's often shown with a single
F. The online Collins, for instance, gives
defo as the primary spelling, with
deffo as a variant.
I'll add both versions as rare words.
For the benefit of those who haven't encountered this term, a recent example from a travel article in the
Scottish Sun:
But there is no doubt the highlight is the MASSIVE pool. It snaked round what seemed like acres of grounds, and was surrounded by luxuriously-padded loungers. Deffo no fighting for a sunbed on this trip, that’s for sure.
One thing that's unclear is where the word started.
Green's Dictionary of Slang says
deffo was originally Irish, though the earliest example given, from 1966, is from a book by Thom Keyes, a US-born author raised in the UK. The OED has a 2016 entry for
deffo, which it says originated in Australia, backed up by a 1940 example from the
Sunday Times of Perth, Western Australia:
Does the girl lend her escort money to take her out on the understanding that it's paid back on Friday?..But deffo!
The Macquarie has
deffo for the definitely meaning, and
defo meaning defamation. This latter usage I had not noticed before, but it does seem to be a well-used bit of lawyer's jargon, in Australia at least. For example, Buzzfeed reporter Laine Sainty tweeted the other day, "The Osman Faruqi v Mark Latham defo case has settled ..."