Larry Csonka celebrated the Steelers' loss like a BOSS on Monday ... breaking out a stogie and booze after Washington helped ensure the 1972 Dolphins would remain the NFL's only undefeated team for at least one more season.
"Tonight, I'm a Washington fan!" the legendary Miami running back said as he puffed his cigar.
Of course, Csonka and other Fins players from the '72 team that recorded the NFL's only perfect season have made it an annual tradition to throw a little party every time the league's last unbeaten loses.
Usually, it's a petty tweet or somebody poppin' bubbly ... but Csonka really went the extra mile this year!!!
The 73-year-old -- who was inducted into the Pro Football Hall of Fame in 1987 -- stood in front of his TV while Washington wrapped up the win and he sipped his booze and gave out a cackle.
"Whohoooooo!!!!" Csonka wrote as WFT was sealing the 23-17 victory over Pitt. "Burn that clock out!"
So far, no other public celebrations from Csonka's old teammates yet ... but we're pretty sure it won't be too much longer until we hear from Mercury Morris.