Which fallacy is this?

I see no direct fallacy, but it borders on many, especially depending where this conversation is headed.

Why not "to define wrong, we need to first know what is right?" One can argue that this is an example of Circular Reasoning . This is a case where we have two opposites, let's say A and Z. Together, they make up a single concept. What we are really asking is to define the concept. Without knowing the concept, we cannot properly define A or Z. If we can't know what is Z, then we can't know what is A. If we can't know what is A, we can't know what is Z, and we're screwed.

What's "wrong"?
The opposite of "right."
What's right?
The opposite of "wrong."

Let's suppose we play this game and do as the person suggests and define "right" based on what we know about "wrong." What we end up with is the appearance of understanding, but actually a. Morality is perhaps the #1 concept that very few people understand yet pretend to, so they try to define it with examples (e.g., "It's wrong to torture a baby for fun."). By Appeal to Extremes , we once again give the illusion that we know what we are talking about, yet without an understanding of the concept itself, we are lost when it comes to determining right or wrong in more moderate cases.