These are two words that I do see mixed up on occasion. It might just be typos, but I figured it’d be better to explore than to not. These two have very different meanings.
EXAMPLES
Rusty defiantly stood at the window, waiting for the threatening kitten she saw in the glass to arrive again.
Rusty definitely was high on cat nip.
Adrian J. Smith 
haha they’d be even more mixed up in Scotland, where definitely is pronounced defiNATEly!
lol I’ve heard some people in the US pronounce it that way. It would be more confusing!!!