In Argumentation Theory, there are concepts called fallacies. They are mistakes of reasoning, discovered and elaborated by scholars over the ages, known to have been used in discussions and debates as an attempt to support an incorrect claim.

One of these fallacies is the Ad hominem.

It is usually used in discussions where the counterargument is not properly addressed, which can cause a cascading effect that might destroy the discussion completely, like what you can see at the bottom of Paul Graham's Hierarchy of Disagreement:

A discussion starts refuting the central point, but can go down to Responding to Tone, Ad-Hominem and Name Calling.

Most people believe that ad-hominem is just about attacking the physical characteristics or the authority of the author instead of focusing on the argument. However, ad-hominem can manifest itself in many subtle ways.

For example, in software engineering, teams argue about abstract technical decisions all the time. Eventually, a situation might come up where someone provides an idea or argument that is contradictory because that person is known to belong to a group that has made many mistakes in the past or is a junior developer. The instinct is to ignore that person's ideas merely because of those past experiences, arguing that the idea is irrelevant based on the credibility of the person that is making it, questioning his or her capability of delivering a right answer.

That is ad-hominem.

More specifically, a Tu quoque if the focus is on a single person, or a Guilt by association if the focus is on the group that person belongs to.

Ad-hominem is when somebody tries to criticize the argument but instead criticizes the person. Whether the attacker is correct in criticizing the person is irrelevant, for the point of ad-hominem is focusing on the person, not the argument.

Ad-hominem is to ignore somebody else's argument or idea because of the person that is making it, instead of the content of that argument or idea.

In this case, it is the responsibility of all parties to put the arguments or ideas on the table and argue against the content of those, not against who is defending or attacking it.