When speaking to your boss you always need to maintain a certain level of professionalism.
Ultimately, your job is a place you should be on your utmost best behaviour.
A place you always show the very best version of yourself.
So saying something inappropriate of downright stupid at work is a no, no.
The best advice I can give is to always take a deep breath before you speak to your boss and gather your thoughts.
If you think you may regret saying it, then you probably will.
Apart from the obvious such as profanity or insults here are 9 things you should never say to your boss.