The Oakland Raiders head coach was finishing off a team meeting when he instructed the players to serenade GM Mike Mayock.
What ensued was pure, hot garbage -- insanely terrible. Embarrassing, actually.
Of course, there was other important stuff that went down on HBO's "Hard Knocks" on Tuesday -- from Antonio Brown's frostbitten feet to his helmet drama ... to tight end Darren Waller's battle with substance abuse.
But, the worst part of the Raiders' camp ... Gruden singing.