"What should I deprioritize?" I've known a couple supervisors who hate this response, effectively responding in turn "nothing!" (Fortunately I did not work for them myself, just heard them complain about it.)
Of course this is ridiculous. When everything has the highest priority, there's no priority between the tasks.
The solution? Use my judgment and ignore the marked priority. The boss was going to be an ass either way.
So if I agree, I say "yes" and I do it. If I disagree, I say "yes" and I don't do it. Usually, when I disagree, that's because it is not as important as they think, and therefore they don't have to come ask a second time: they just forget about it.
What kind of world do the people who write this stuff live in?
the response is: list out your priorities and the impact of not doing them long with a recommendation of moving forward
... isn't that YOUR job? This especially sucks when it happens regularly and it means you're not spending any time on the prioritized work, just working on setting priorities.
Then, at least you know that you are dealing with a jerk.
If you were just saying no because you didn't -want- to do it. Well, then your manager is calling your bluff, and ya, you need to find another way to wiggle out of it. Tough luck.