The beef between Antonio Brown and JuJu Smith-Schuster is BACK ON ... AB just went after the Steelers star once again, calling him "boo boo" and a "bum."
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.