What is the term for dismissing someone’s opinion based on their lack of support for a certain idea?

The correct term for dismissing someone's opinion based on their lack of support for a certain idea is indeed "Ad Hominem." This logical fallacy occurs when an argument is rebutted by attacking the character, motive, or other attribute of the person making it, rather than addressing the substance of their argument. In this context, by focusing on the individual's lack of support instead of their actual viewpoints, the argument does not engage with the reasoning behind the opinion itself.

Recognizing an Ad Hominem fallacy is crucial because it highlights a common error in discourse where personal attacks overshadow rational debate. This can create a barrier to constructive dialogue and understanding, as it diverts attention from the key issues at hand.