Fivio Foreign has been quietly wilting away in New Jersey jail since January on multiple charges ... but his attorney says it's the weakest case he's ever seen!!!

TMZ Hip Hop spoke to Adam Lustberg, who's been repping Fivio for years and tells us he expects the Brooklyn drill pioneer to be released in a couple months.

We obtained the docs ... Fivio is accused of pointing a gun at a woman who was simply asking him for a car jumpstart on New Year's Day in Edgewater, NJ.

According to the police reports, Fivio pointed the gun at the woman and said, "If I see you parked in front of this building again there is going to be a f****** problem."

Cops say witnesses recognized Fivio right away, dialed 911, and later gave statements to officers.

Edgewater Police also say Fivio was seen on surveillance footage from the apartment complex, which they say corroborates the alleged victim's story.

Fivio was arrested on January 18 and hit with 5 charges, including unlawful possession of a weapon, terroristic threats, and aggravated assault.

Lustberg says police never recovered a gun from Fivio, and he believes the case against his client is one of the flimsiest cases he's seen in his 21 years of practicing law.

The BK rap star's new collab just dropped with Caldwell -- but he's not around to promote it!!!