Even if you don't know Latin, you probably know what ad hominem means. (It means "against the person".) You've certainly experienced it, either in your personal life or in the media. "Such-and-such" can't be true (or must be true) because "So-and-so" said it. Rather than deal with the content of what was actually said, an ad hominem response--or argument--seeks to discredit the idea based solely on the person making the statement.
Even otherwise silly or even nasty people can speak the truth. As the old joke goes, in the Bible, God speaks to Balaam through the mouth of an ass--and he has spoken to me the same way many times. Sometimes, I must admit, I have been the ass.
When we hear something with which we disagree from someone whom we think lacks credibility--or whom we simply don't like, the temptation is to discredit it based on our disdain for the person. We may voice our convictions in much the same way Lucy does in the cartoon. But, the illogic of doing so is obvious.
In our contentious society today, it is too easy--and too often simply wrong--to say "He's either lying or he's stupid" when someone makes a public pronouncement that later proves to be wrong. That binary approach leaves out many other, much more likely, possibilities. The person may have been misinformed, he or she may have been giving their honest and best opinion based on the information available to them at the time, the person may have been legitimately confused, etc. To imply that there are only two possible reasons for a person to be in error (intentional deception or stupidity) is simply not the way most people live their lives most of the time.
I have found, and I'll bet you have too if you think about it, that most people most of the time are doing the best they can. I hope that's true of you. And, until you prove otherwise, that will be my assumption about you. I hope it will be your assumption about me, as well.
So, let's be done with the all-too-convenient and almost always wrong ad hominem attacks. They do nothing to advance truth or love.