EXCLUSIVE

YFN Lucci changed two words of a lesser-known rapper's song title and lit it on fire by making it a commercial hit and pawning it off as original ... this according to a new lawsuit.

The Altanta rapper -- who's said to be dating Lil Wayne's daughter -- just got sued by another MC named Rackboy Cam ... who claims YFN jacked his song "Everything Be Lit" and warped it into the wildly successful "Everyday We Lit" with negligible differences.

According to the docs, obtained by TMZ, Cam says his song came out first in 2015 and that YFN's 2017 track is virtually the same song, especially when it comes to the hook -- namely, "Everyday we lit" vs "Everything be lit," sung in a similar melody.

Cam also claims that YFN lifted 2 out of the 3 elements Cam brags about in his song -- his chain, his whip and his bitch ... saying YFN raps about jewelry and women.