I am sick and tired of people misusing the term ad hominem.
First, let's explain some things. An argument supports or fails to support a claim. A good argument is defined as an argument that supports its claim. A bad argument or fallacy is an argument that fails to support its claim.
Ad hominem is a type of logical fallacy. First, let's examine two statements:
You are a meanie mean-head
Your claim about marine biology is wrong because you are a meanie mean-head
All too often, I see people pointing at statements like statement number 1 and declaring "That's an ad hominem! Ad hominem!" Statement one is not an ad hominem fallacy. Indeed, it is not a supporting argument, and thus cannot be an ad hominem fallacy, nor any other kind of fallacy. Statement one is a claim, not a supporting argument. If someone makes this claim, it is up to them to provide supporting evidence for their claim. If they fail to provide supporting evidence for the claim, then we cannot conclusively say that someone is a meanie mean-head.
Statement two is indeed a supporting argument. This is an example of an ad hominem logical fallacy because whether or not someone is a meanie mean-head has absolutely nothing to do with whether or not a given claim about marine biology is right or wrong. Thus, the supporting argument cannot support the claim (that the other claim is wrong), therefore it is a logical fallacy.
When people identify statement one as an "ad hominem," they are implying that it is false because it is an insult, which ironically is pretty close to an ad hominem fallacy itself. If all negative criticisms are automatically false because they are "ad hominems," then this means we can never criticize anyone for any reason because our criticisms are automatically false due to being criticisms. See the problem? If you use ad hominem in this way, you are making a statement that is utterly incoherent. If statement one is a logical fallacy, then we can't even criticize Nazis for the Holocaust.
If you don't understand what an ad hominem logical fallacy is (or even what a logical fallacy is, as in the case with most people using this term), then please do not use the term. You're only making yourself look worse.