The negation would be "there does not exist at least one person that is better off, or, there exists someone that is worse off." That is the improper use of police power. Indeed, this could be a stylized definition of racism if you defined racism as the theft of life or property from one group to another based on the color of their skin or other social classification. The opposite of Pareto optimization would be something like institutionalized racism.
A person commented it did not answer the question. That isn't true, but I will expand on the discussion.
The negation of the definition of Pareto optimality is above. Either a reallocation has happened but no one is better off, or at least one person is worse off.
For the former to happen, there would have to be a use of police power. If everyone is indifferent then there is no reason for reallocation to happen. Although no one cares, it isn't rational for changes to happen as it is an equilibrium.
An example of this is where a bunch of people are sitting around a table and no one cares where they sit. The only way reallocation of chairs will happen is if someone with power says "okay, everyone shift right one chair, for no reason, just because I have the power to do make everyone do it."
The second case has at least one person worse off. No one would rationally make themselves worse off. That also requires police power.
The problem is that English lacks a good term for theft of goods through police power except in the case of racism.
Note that the definition of racism is not bigotry or prejudice. Bigotry is the strong personal aversion by a person against a group, possibly racial or ethnic but it could be on other dimensions such as religion or politics. Neurological studies of bigotry show that it processes in the area of emotion connected to smell, disgust and emotion and links the group to danger to food. That is bigotry.
Prejudice is simply the fast judgment of a person by observable characteristics and may not be related to race.
Racism is the theft of property or life from one group to another through the use of police power, whether formal as in Title One of the Housing Act of 1947 also known as the Negro Removal Act or informal as when the Klan was permitted to kill or seize property without governmental recourse by the victims. It is the use of police power through acquiesence.
The only requirement of the negation of the Pareto principle is indifference OR at least one person loses.
That could also describe crime as it says nothing about at least one person being worse off.
I don't agree with your negation. It does not follow from de Morgan's laws.
Finally, it could happen due to taxation and reallocation. Those would not be crimes either, moral or otherwise, but we lack a term for it. Libertarians view it as theft for this reason. Most people consider it morally neutral because there if there is no intent to specifically do harm to any group for the benefit of others.
I should point out that we usually lack words for things other than optimums and points of inflection. If you look at $y=-x^2$ there is an extreme value and we call it a maximum. We do not have a name for the other uncountably many points.
Based on your comment, where someone is worse off and no one is better off is called waste. However, for the reallocation to happen on purpose it would mandate irrationality to happen. I do not mean irrationality as a psychologist would mean it. For example, a schizophrenic may have psychotic preferences but they still act rationally on those preferences.
Let me give you an example. Let us assume that I hate you and you have apples that have rotted. You do not care because you can turn them into applesauce and you are indifferent between fresh apples and applesauce. I, on the other hand, hate applesauce.
I offer to trade your rotten apples, which you are indifferent for, for my good apple making me worse off. Although I am indifferent to trade a good apple for a good apple, I am not indifferent to getting a rotten apple.
So I would be acting in an individually irrational manner. There is no word for this but the term of art would be to act in an irrational manner.