I'll tell you. It is definitely "defiantly."
Now, I realize that it is due to spell checkers (and lazy people) but I searched Pinside and the word "defiantly" appears in 640+ posts, and I gave up trying to find posts where it was not used incorrectly in place of "definitely." I would wager that it is used incorrectly 100 times for every time it is used correctly. That's crazy!
The root word of "defiantly" is "defy," a verb. "Defiant" is an adjective built on that, and "defiantly" is an adverb built on that!
Whereas "definitely" the adverb is based on "definite" the adjective.
Now, cannot everyone see that "definite" is not the same word as "defiant?" It would seem so; when I searched for "defiant" I didn't see anyone using that word incorrectly.
So what's so confusing about adding an "ly" to the correct word?
Again, yes, we have spell check to thank for this phenomenon. Start a word with "defi," goof up the middle, add "ly," and spell check will ever so helpfully suggest "defiantly." Which is great, if that's the word you were after. But, for the love of all that is good and holy, don't accept the suggestion when it isn't!
Running this into the ground, it's probably the misspelling "definatly" that is the main culprit. Swap the "n" and "a" and there's "defiantly," which explains the spell check suggestion. Just remember, there is definitely no "a" in "definitely!"
I'm still astounded that anyone would say "ok, yeah, I meant defiantly" when they didn't!
DE_FI_ ANT_ LY
DE_ FIN_ ITE_ LY
I guess all I'm asking is for people to pay better attention. Rant over!