defiant: defined as “boldly resistant or challenging.”
definite: defined as “clearly determined and certain; positive.”
If you were, say, I am defiantly having a good time, then you were boldly resistant to such; whereas, if you were definitely having a good time, then you were certain and positive it was a good time.
It is crazy-making behavior, especially considering that defiant and definite are not pronounced the same, unlike the common mix-up between your and you’re and there, their, and they’re.
Please. Do the right thing. Use the proper word and proofread your work.