Answer by President Donald Trump: What about crooked Hillary? She is the biggest liar. [hypothetical answer].
This exchange about is the essence of whataboutism.
When pressed, a whataboutist will not answer the merits of the question but will instead attempt to accuse the questioner of not recognizing the underlying hypocrisy of the question, even if it is unrelated to the actual answer. It is a variant of the more well-known tu quoque fallacy.