Deffo is a very informal way of saying definitely. You use it when you want to sound sure, casual, and friendly: you’ll deffo come, that’s deffo the one, I’m deffo not going. It is mainly British (and you also hear it in Australian English). Young people say it in chat, in texts, and in relaxed talk. It can work like an adverb (“I’ll deffo be there”) or, more loosely, like an adjective (“That’s deffo true”). It is playful on purpose — the clipped, almost cute shape is the point. You would not write it in an exam, an email to a teacher, or any careful piece of work. There, the full word is definitely. Americans more often say definitely, or other slang; deffo will sound British.

一定、絕對（英式口語，等於 definitely）