EXCLUSIVE

Lindsay Lohan wasn't spewing BS on "Jimmy Fallon" Thursday night -- talking about a "Mean Girls" reunion -- TMZ has learned, it's been in the works for weeks.

FYI, 2014 is the 10th anniversary of the film's release -- and sources tell us, co-writer Tina Fey has been leading the charge for a reunion to commemorate the occasion ... wrangling the entire cast over the last month.

And Lindsay was apparently the last to know about it -- she went on "Fallon" Thursday talking about how she had just run into Tina who mentioned the reunion idea to her ... and Lindsay was into it.

It's unclear how the reunion will come together -- Jimmy suggested hosting it on his show -- but we're told it could also materialize as a comedy sketch ... maybe on an awards show.

Sources close to Tina tell us, several prominent cast members are already on board -- Amy Poehler (Mrs. George), Jonathan Bennett (dreamy Aaron), Rajiv Surendra (Math Enthusiast / Bad-Ass MC), and Daniel Franzese (Too Gay to Function).

Pretty awesome. Now get in losers. We're going shopping.