That's Sgt. Slaughter on the death of his rival turned stable partner, and most importantly close friend, The Iron Sheik ... who passed away at the age of 81 on Wednesday.

TMZ Sports talked to the WWE Hall of Famer about his friend's passing ... from Sheik's impact on the sport (and some of its biggest stars like Hulk Hogan) to how they met decades ago.

"I met [Sheik] when he was going through the training camp in Minnesota with Verne Gagne, along with Ric Flair," Slaughter explained, talking about his first interaction in the 1970s with the Iranian-born wrestler.

FYI, Gagne was a trainer and promoter who ran the Minneapolis-based American Wrestling Association (AWA).

Sgt. says he became fast friends with Sheik and the guys even worked together at his dad's roofing company.

The Iron Sheik was one of the greatest heels pro wrestling has ever seen and, although he was playing a character, fans take this seriously. We asked Slaughter if the hate ever got to Sheik.

"Never difficult [on Sheik]. It fueled the fire. It just fueled the fire," the HOF told us.

Slaughter also talked about Sheik's love for his family -- his wife and kids.

"He was a character. I was a character. But we both had families and we loved our families."

Tragically, Sheik lost one of his daughters in the early 2000s ... and despite missing his friend, Sgt. Slaughter, fighting back tears, says he's happy knowing he'll finally be reunited with his child.

"God bless him. He's finally gonna see his daughter that he lost. Love you Sheik. Love you forever."