During his recent Humor & Harmony Weekend, 50 couldn't remember Desiigner's name while chatting with Gillie and Wallo ... but remembered the Brooklyn native only had one chart-topper in the chamber when they sat down together circa 2015 as "Panda" was dodging comparisons to Future's sound.

Desiigner wasted no time putting together a reaction record -- which he captioned "POV: Another Hit" on Instagram, despite the song sounding pretty unfinished.

He threw on a Queens baseball cap to troll 50's NYC borough and had someone film him from the backseat of a car as he rapped the song lyrics from his phone.

After mumbling through the opening lines, he gets to the juicy part ... "N***as talking that 50 Cent / I be talking 50 mil ... I got signed to Kanye lil n***a, you was never worth a deal ... Desiigner always keep it real for my bitches."

It's interesting that Desiigner chose to leverage his Kanye association after labeling him crazy during their 2019 public fallout ... but even more so that he's rapping after quitting hip hop following Takeoff's death.

50 is currently in author mode, promoting his new novel "The Accomplice" -- but maybe he'll switch up and find a studio to redirect some bars Desiigner's way.