If you haven't been following on X this week ... the former Washington head coach and his ex-QB have been at each other's necks ever since Gruden sent off a comment during the Eagles vs. Buccaneers playoff game Monday night.

Jay wrote on his X page, "If I ever put a QB through what Philly is putting Jalen [Hurts] through, I apologize." RGIII -- who played for Gruden in 2014 -- quickly clapped back, writing, "Say WHATTTT???" ... and the two then proceeded to go at it.

The guys threw a few barbs back and forth ... and just when it looked like the squabble had ended -- Griffin III revived it on his podcast Thursday evening.

During his show, the 33-year-old accused Gruden of asking him to call out his teammates following a game in '14 -- before he claims the coach turned his back on him.

That's when Gruden went nuclear ... and shot out a tweet where he insinuated no one in the NFL liked RGIII.

"You weren't good enough," Jay wrote. "Kirk [Cousins] was better."

"Cleveland didn't want you." he continued. "Baltimore [didn't] either. Quit blaming me."

Jay deleted the post a short time later ... and replaced it with a video of RGIII running alongside the Seahawks' live mascot during pregame festivities in Seattle earlier this season.

"Go race a pigeon," he said.

RGIII has since responded with yet another dig at Gruden -- writing, "The Pigeon name is more respected than your last name."