1. To do something rebelliously; often disobeying authority or some other source of external social pressure.
2. Common mis-spelling of definitely, often arrived at when a person types definatly or definately into a spell-checker, and the spell-ckecher changes it to "defiantly".
A. Go away you moron!
B. (defiantly) No.
A. Do you want to work 30 hours' unpaid overtime this week?
B. I defiantly don't.
A common misspelling of "definitely". Its origin is unknown.
We defiantly need to do that again!