Unclear why Brown decided to reignite his war with his former teammate ... but he went after the receiver HARD Monday, writing on Twitter, "Boo Boo Shoester was ready under 500 U Bum learn some Respect"
Of course, AB and JuJu really got into it earlier this year after Brown was traded from Pittsburgh to Oakland ... with Brown shading JuJu for his 2018 fumble issues.
JuJu fired back on social media by writing, "Crazy how big that ego got to be to take shots at people who show you love! Smh"
But, it seemed JuJu was ready to bury the hatchet on it all ... 'cause Smith-Schuster told us back in April that Brown is "my guy."
Yet, clearly, the beef still remains ... 'cause AB only needed about 24 hours after the Steelers' season ended to take a shot at JuJu's down stat line this year (just 552 receiving yards in 12 games).
There's more ... AB also appeared to take a shot at his former quarterback in Oakland on Monday, getting in a dig at the possibility Jon Gruden might cut bait with Derek Carr in 2020.
"Tell Derek Carr I’ll buy his house in Vegas," Brown wrote.
As for DC ... he seems confident he'll still be in the Silver and Black when they move to Vegas next season, telling media members Monday, "I had some good meetings with my coaches. I'm looking forward to Las Vegas."