Before their first fight in 1971, Muhammad Ali made sure that no one could possibly misinterpret his feelings towards heavyweight champion Joe Frazier.

Frazier was too ugly to be the champion, Ali said. He was too dumb. Maybe most injurious, Ali called him an “Uncle Tom,” sticking him in a camp with the white American establishment that had tried to ruin and imprison Ali for his refusal to be drafted during the Vietnam War.

“He’s the other type of Negro; he’s not like me,” Ali said of Frazier at the time. “That’s what I mean when I say Uncle Tom. I mean, he’s a brother. One day he might be like me, but for now he works for the enemy.”

These were powerful words, and they stung Frazier. They had a lasting impact, too, both in terms of how the public saw Frazier and in terms of how he felt about Ali, his greatest antagonist. No matter how his trainers and management tried to convince him that this was just Ali selling their fight by drawing a cultural line that would force everyone in America to choose sides, Frazier took it all personally.

“He had me stunned,” Frazier said later. “This guy was a buddy. I remember looking at him and thinking, ‘What’s wrong with this guy? Has he gone crazy?’ He called me an Uncle Tom. For a guy who did as much for him as I did, that was cruel. I grew up like the black man – he didn’t. I cooked the liquor. I cut the wood. I worked the farm. I lived in the ghetto. Yes, I tommed; when he asked me to help him get a license, I tommed for him. For him! He betrayed my friendship. He called me stupid. He said I was so ugly that my mother ran and hid when she gave birth to me. I was shocked. I sat down and said to myself, ‘I’m gonna kill him.’ OK? Simple as that. I’m gonna kill him!”

As Frazier’s son Marvis would later recall, the night that Frazier fought Ali for the first time, in a star-studded event at Madison Square Garden, he first knelt down to pray. When Marvis later asked his father what he said during that prayer, Frazier replied that he had asked God to let him kill Ali “because he’s not righteous.”

Frazier would win that bout, dubbed the “Fight of the Century,” via unanimous decision. It shattered previous viewership and earnings records, with an estimated 300 million people watching the fight across the globe. The rivalry spawned two more fights, including the “Thrilla in Manilla,” where Ali hyped the fight in part by mercilessly taunting “the gorilla” Frazier, playing on racial stereotypes that were centuries old. Frazier never forgot it.

Decades later, when Ali lit the torch at the 1996 Olympic Games in Atlanta, Frazier criticized him as a draft dodger and said he wished he could have pushed Ali into the torch after he’d lit it.

Even as Ali suffered from the ravages of Parkinson’s disease, Frazier showed him no sympathy. It was proof, Frazier said, that he’d been the real winner of their three fights. Ali didn’t have Parkinson’s, Frazier told people. He had “left-hook-itis.”

“Truth is, I’d like to rumble with that sucker again—beat him up piece by piece and mail him back to Jesus. … Now people ask me if I feel bad for him, now that things aren’t going so well for him,” Frazier said. “Nope. I don’t. Fact is, I don’t give a damn. They want me to love him, but I’ll open up the graveyard and bury his ass when the Lord chooses to take him.”

Ali and Frazier weren’t the first fighters to discover that words can hurt, even in the hurt business. As Khabib Nurmagomedov and Conor McGregor showed us at UFC 229 on Saturday night, they were also far from the last.

Jack Dempsey used to drum up business for himself in mining town bars by announcing to the room that while he couldn’t sing and couldn’t dance, he’d beat any man in the house in a fist fight. This was itself just a slightly modified version of bare-knuckle boxing champion John L. Sullivan’s famous catchphrase: “I’m John L. Sullivan, and I can lick any son of a bitch in the world.”

Heavyweight boxing champ Tommy Burns was known as a smart (though undersized) fighter who would get in opponents’ heads with his verbal jabs. Then he met Jack Johnson, who wasn’t the least bit shaken by his words, merely laughing at him in the ring as he asked him: “Poor little Tommy, who told you you were a fighter?”

“Burns started it by calling me a yellow cur and other language which it would be impossible for me to report,” Johnson wrote later, as documented in Geoffrey C. Ward’s excellent biography of Johnson, “Unforgivable Blackness.” “I only kidded him in a nice way, but he used the other sort of language … If I had killed Burns for the language he used to me I would have been fully justified.”

This is not so different from the argument Nurmagomedov made after his actions at UFC 229. While he apologized for leaping out of the cage and attacking McGregor’s cornermen, he also placed much of the blame on McGregor for inciting that level of rage with all his pre-fight antics.

“Media change a little bit (of) MMA,” Nurmagomedov said. “It’s a respect sport; this is not trash-talking sport. This is respectful. I told you guys, ‘I want to change this game.’ I don’t want people to talk (expletive) about opponent, talk (expletive) about father, like religion. You cannot talk about religion, you cannot talk about nation. For me, this is very important.”

But trash-talking has been a part of the fight game for about as long there’s been one. It plays a unique role in prize fighting because of the unique nature of the sport itself.

Basketball players don’t get so mad at each other that they start playing hockey. When someone goes too far in football, it doesn’t prompt the opposing team to start up a baseball game.

In other sports, fighting is an end point. It’s the sign that rules and decorum have broken down.

But in fighting, that’s where things begin. In this sport, it’s not a question of whether or not the opposing parties will get so angry that they come to blows. That part is a given. So when they start winding one another up at a press conference, it only invites us to consider how it’s going to feel and what it’s going to mean when they’re finally allowed to hit one another for money.

And, don’t kid yourself, the money is a big part of it. If the trash talk makes us care, or if it builds anticipation for the inevitable violence, it leads to more money for the participants in the end. That’s how it worked for Ali and Frazier. It’s also how it worked for Nurmagomedov and McGregor.

We’ve proven over and over that we will pay more to see two people fight if we believe that they really hate each other. If their conflict is a stand-in for a larger cultural or political fight, even better – though also more volatile.

With UFC 229 in particular, you take away all the talk and the hype and the personal animosity, and you likely end up with a slimmer paycheck at the end.

Gallery Khabib Nurmagomedov def. Conor McGregor at UFC 229: Best photos view 15 images

But where should the lines be drawn? For Nurmagomedov, McGregor’s remarks about his father, as well as his cultural and religious identity, seemed to be a step too far. Then again, former UFC champion Anderson Silva endured arguably worse treatment before his two fights with Chael Sonnen, and somehow he never felt the need to carry the fight to Sonnen’s entourage on the floor of the arena once the bouts were won.

Fighting is not only an individual sport, but also a deeply personal one. It probably shouldn’t be a surprise when the talk to build it up – weeks of talk, sometimes, far more talking than actual fighting – gets personal as well.

Drawing a line virtually guarantees that eventually someone will cross it. And the history of the fight game tells us that the harder it gets to shock and surprise us, the further fighters and promoters will go just to ensure they do it – even if it ultimately leaves some scars.

As Ali and Frazier showed us, sometimes those scars are permanent. Some lines, once crossed, can’t be uncrossed. Not even if it was all just intended as salesmanship and good business.

For complete coverage of UFC 229, check out the UFC Events section of the site.