I agree that she probably didn't know it meant "for sure". She probably heard the "F" and an "izzle" and felt like you were being mean to her.

I would just be careful about using slang. Everyone deserves respect in the workplace. Whether they are above you or below you. Slang can be interpreted as a "one-up" against some people, which indicates a "looking down".

Also, if they don't get it, they feel stupid or embarrassed.

Some people use slang for that very purpose, too!

It's just best to stay away from it unless you have a very good friendship with the person you are talking to.

I don't know what the supervisor's deal was. It could be that your supervisor is not very good at interpersonal relationships. They may be a GREAT supervisor when it come to the job at hand, but CLUELESS when it comes to interpersonal relationships. I've had bosses that were so dramatic, it is a wonder they could keep their jobs. Best to stay professional with them, too. And that means not calling them after hours.