I guess I'm not completely clear on what "What is this _" means as a rhetorical question. I can tell it has a connotation of mild horror and mock exaggeration. For the purpose of stating something is unjust, should it be "What is this injustice?" or "What is this justice?"
-
On the face of it the answer is obvious. But perhaps the sentences immediately before or after it might explain why you have a problem. That is why questions are supposed to give a clear context. I can tell certain things. The word ‘this’ presupposes that somebody, possibly a defendant or plaintiff in a lawsuit has complained of an injustice; or it might be a suppliant before a magistrate or ruler. But we need a context.– TuffyCommented Apr 10, 2020 at 23:09
2 Answers
To make a long answer short, if you were to write "Doing XYZ will result in a big injustice to the people of ABC", it would be perfectly idiomatic to follow that with the rhetorical question "What is this injustice?" and then an explanation.
People use "What is this?" to express their dismay at something, for example:
"What is this mess?"
Depending on the context of the rhetorical question, I would say "What is this injustice?" in the same way I would say "What is this mess?"--to express my dismay. However, if the speaker wished to imply that "this" was not true justice, he/she might say:
"What is this, justice?" (Note that the use of a comma changes the tone and meaning of the sentence.)