Jon Gruden ... clearly wasn't invited to any birthday parties as a kid -- because his rendition of "Happy Birthday" on "Hard Knocks" sounds like it's the FIRST time he's ever sang it.
Seriously ... it's terrible. Like worse than Hunter Renfrow bad.
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.
Seriously, bro. Bad.