Did you install a word-filter that changes every instance of 'definately' into 'defiantly'? I just ran across five separate posts that get this wrong, it's beginning to look suspicious!
Indeed, the correct way to misspell definitely is definately or definetely. I guess definetly, definatly, definitly also are good ways to misspell definitely too. It definitely isn't defiantly though.
Another culprit here for misspelling 'definitely' as 'definately' although I was sure that this word had migrated to some sort of american alternative spelling like 'color'. Unfortunately it was only my poor english skills at fault.
http://www.dailyrecord.co.uk/news/editors-choice/2009/06/15/definately-the-most-misspelled-word-in-the-english-language-it-should-be-definitely-86908-21441847/ Because the dailyrecord is the gospel of undeniable fact! I always cringed when I heard people pronounce diffuse as 'defuse' :p