HOW DO YOU SPELL DEFFINATELY??????????

Alright all you intelligent English majors out there, I need some help, I find I use the word deffinately a lot in my life, through e-mails, blogs, journal writing. One day I think I tried twenty different ways to spell it and it kept coming up with that stupid red line underneath it saying it was spelled wrong. So please help me to correct my terrible spelling, or grammar, maybe it is not even a word.