How to loosen a grinder disk that was tighted to tight?

Originally Posted by injunjoe

That nut is made to be turned with equal pressure from each side, as to not put pressure on the threads.
With a pair of Visegrips the pressure is applied to only the sides of the nut, thus making it oblong, making it even tighter against the threads on the shaft.

For what it was worth, it worked for me. You don't want to make it super tight and crush it, just enough pressure to bite it and turn.