Of course, we're referring to Paris Hilton's oil heir buddy, Brandon Davis, once calling her a "fire crotch" gal -- a line that actually made its way into the rebooted flick ... which LL apparently had no idea about -- at least that's the indication from her team's response.

Lindsay's rep told The Messenger ... "Lindsay was very hurt and disappointed by the reference in the film." The outlet also reports that Lindsay was surprised last week when she watched the movie for the first time at the premiere.

It's pretty shocking that the line even made it into the film -- y'know, considering Tina Fey wrote it ... not to mention the fact that Lindsay herself cameos in the flick as well.

On its face, it seems she had no idea it was coming ... meaning she either didn't receive the full script, or didn't peruse it. Ya gotta imagine that if she saw this ahead of time ... she would've talked to Tina about it, because LiLo has famously spoken out about the dig.

The context of how it's used in the new movie features none other than Megan Thee Stallion -- who shows up as one of many people singing the praises of the new popular girl, Cady (once played by Lindsay), as they flee Regina George. MTS says "fire crotch is back."

You'll recall, the way it was originally used dates back to a TMZ video from back in 2006 ... when Davis was being a bit of a "mean girl" himself in talking trash about LiLo at the time. Like we said, Lindsay's been fairly vocal about how hurtful that was back then.

That's why Tina's decision to include it in the movie -- even in jet -- is shocking ... and now even more so with LL speaking out like this. Tina gushed about working with Lindsay, so you know she loves her  ... perhaps this was just an innocent mistake or an oversight.