WBC heavyweight champion Tyson Fury took aim at Oleksandr Usyk over his recent performance against mandatory challenger Daniel Dubois.
Last month in Poland, Usyk retained the IBF, IBO, WBA, WBO titles with a ninth round knockout of Dubois.
However, there is ongoing controversy over the fifth round - when Dubois landed a body shot and Usyk went down in pain for several minutes.
The referee ruled it as a low blow, but there are observers who felt the punch was legal.
Fury is among those who felt Dubois connected with a legal punch and Usyk should have been counted out.
Usyk would eventually recover and continue to fight. He dropped Dubois in the eight and once again in the ninth for the stoppage.
"I thought he lost [to Dubois] and he quit so he's a little, quitting [coward]," Fury said to Sky Sports. "And once you quit once you'lll always quit, just like we saw with Dubois. He quit too, so that was two quitters fighting each other.
"It's tough to say somebody's quit but Usyk quit with a body shot and rattled around the floor like a little [coward] for five minutes and the other one quit with a jab. But the truth is the truth and that's it.
"I thought [Usyk] was a warrior, I thought he was a true little fighting man. But he's clearly not, he's just a little wussy. I've been hit full-on, right [below the belt] with the hardest punches you've ever seen, I didn't roll around like that. It is what it is, you're a fighting man or you're not and if you quit once, you will always quit again. My father told me that when I was a child. If you spew it once you'll always spew it and that's the facts of the matter. End of."