Jay Gruden just took his beef with Robert Griffin III to a whole 'nother level ... firing off a shot at his former quarterback that was so petty, he actually felt the need to delete it.
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."
The tiff has now hit a pause ... but given the way everything's gone so far this week -- it seems safe to assume it'll be reignited sooner than later.