Hilchos Lashon Hara chapter 5, section 2

It is forbidden to put someone down for lacking good qualities – in wisdom, in wealth, and the like.

For example, with wisdom: telling people that so-and-so is not smart. Of course, if it is a lie, it is a big sin and is called motzi shem ra (spreading a false bad name). But even if it is true, it is still lashon hara, because lashon hara applies even to the truth.

Saying someone is not smart can cause real harm. If they are not married yet, it may stop them from finding a match. If they have a business, no one will want to be their partner. If they are a rav in Israel, besides the lashon hara, it causes them to lose income, because people will not come to them for a din Torah (Torah ruling) or a compromise.

It might even cause them to lose their position because of this lashon hara. Then their livelihood, and their children's, will be on the head of the one who spoke, because the lashon hara truly took away their living. This is also called shaming a Torah scholar, and Chazal (our Sages) said there is no cure for that wound.

This also badly weakens keeping the Torah. Even if the rav warns people about a mitzvah, they will not listen, because they already think of the rav as someone who is not wise.