We just got Thug's latest mug shot from his arrest Saturday in Brookhaven, GA. As we first reported ... Thug is facing a felony drug possession charge ... and now we know how much.

Law enforcement tells us he allegedly had 2.5 oz of weed on him, and cops found the stash during a routine traffic stop. We're told Thug was initially pulled over for over-tinted windows.

As for why he's still in jail ... the case is still being reviewed by the D.A., and no bail's been set.