Pope Leo XIV's brother will need to cop to using some unsavory language at his next confession ... 'cause he called Nancy Pelosi a bad word in a resurfaced social media post.

The pope's older brother, Louis Prevost, is a regular poster of political content on his Facebook page ... much of it from a more conservative viewpoint.

While there's nothing wrong with sharing a political opinion, one particular post did raise some eyebrows online ... a post where Prevost referred to Pelosi as a "dumb c***."

Here's the deal ... the post is a clip of Pelosi in the mid-1990s discussing U.S. and China trade -- pointing to the trade deficit between the two sides, as well as the sizable difference in exports and tariffs.

The caption -- not written by Prevost -- reads, "These f***ing liberals crying about tariffs is just unreal. Do they not know that there is a thing called video? Just listen to what this drunk c*** has to say In the mid 90’s long before her husband had grindr dates."

Again, Prevost didn't write the caption ... but, he did reshare the video and caption to his own Facebook -- seemingly indicating he shares the sentiment.

BTW ... Pelosi's received communion at the Vatican before, so there's a chance, she's going to meet Pope Leo -- awkward!

We've reached out to Louis about his post ... no word back yet. He appears to be an ardent Donald Trump supporter BTW ... and, he told the New York Times that he expects his brother to be more conservative than Pope Francis -- though Pope Leo XIV has shared liberal sentiments to his own social media in the past.