I am still coming to terms with the fact that this post needed to be written. How do people not know these are different words?
Apparently, though, it’s an all-too-common problem. Social media is littered with posts where someone has answered with “Defiantly!” when what the responder really meant to say was “Definitely!”.
It happens in my own conversations several individuals on a regular basis. In fact, it happened again just yesterday, so I took a screenshot with this blog post in mind.

I don’t know whether autocorrect is to blame, presumably as a result of poor typing, or if it’s just plain old-fashioned ignorance. The answer to that probably varies from one perpetrator to another, but either way, continuing to mistake one for the other is inexcusable.
Definitely means “for sure” or “absolutely”.
In fact, those are excellent choices for anyone who wants to agree with something, but doesn’t actually know how to spell “definitely”.
Here’s a hint, though.
Definitely even sounds exactly as it is spelt: def-in-it-ely.
It’s phonetically straightforward.
Therefore, anyone trying to spell it should get to the ‘a’ in ‘defiantly’ and know they’re making bad choices.
Defiantly is a word most often used by parents or teachers describing the way in which a child refused to do as they were told.
Examples:
The child sat in stony silence, arms crossed defiantly.
“No!” Robin yelled defiantly, “I won’t apologise for being a grammar snob!”
You get the idea.
It’s definitely in your interests to get this right.
Make good choices, people… please?