The only thing that is over the top in this situation is the coffee that spills out of your cup. It sounds like your boss has had his fill of this problem, and your best move is to heed his warning.
With a couple of incredibly simple steps, you can end this problem. First, get a cup that is essentially spill-proof. If you knock it over, a miniscule amount comes out. Secondly, how about placing the cup of coffee out of harm's way? Set it off to the side and place it back in the same spot after you have had your sips.
The fact that your boss is so incensed over this matter sounds like there may be more than coffee that is getting him heated. Take a look at your overall performance and the feedback he has given you over the past few months. This whole coffee issue could be pointing to a broader pattern of insubordination.
The best step is to stop fighting your manager over this and recognize that real trouble is brewing here. It's time to wake up and smell the coffee.
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