Another mispelling of definitely almost as popular as the really dumb mispelling definately. definetly is also stunningly common, more than definitly. Hard to believe, even definatly is used quite often.
When I wrote definetely I was definitely thinking of #define and thus I must be a computer geek.
The modern mis-spelling of definitely seen in almost every modern example where a spell checker was not deployed, most commonly in internet chat, probably from its mis-pronunciation by modern youth and/or being confused with unfortunately.