Khabib is breaking his silence on Conor McGregor's ugly comments about the undefeated fighter's deceased father ... saying "The Notorious" flat-out has a bad heart.

Nurmagomedov joined "Hotboxin' with Mike Tyson" (and Henry Cejudo) ... when the topic turned to McGregor's now-deleted Tweet about Khabib's late dad, Abdulmanap.

"When [Conor] talked about this, only evil can talk about your father, wife, kids, religion. If you’re normal human, you’re never going to talk about this stuff," Khabib told the combat legends.

"For me, I think he post this tweet while drunk too much or something. Then the next day, he always delete these tweets. When he become normal life he says, ‘Oh, look what I did.’ Then he delete. This is my opinion what he do all the time."

In case you missed it (Conor deleted the message not long after posting), McGregor tweeted ... “Covid is good and father is evil?”

That was presumably in response to Khabib tweeting "Good always defeats evil" after Dustin Poirier beat McGregor in their trilogy fight last month.

Nurmagomedov had more to say about Conor, digging into his past mistakes ... including when he socked a man in a Dublin bar for turning down a drink of his Proper 12 whiskey.

“When someone is not with us, he is not even alive, this shows you what you have inside, this shows, like, how dirty you are. When you are one of the best in the world, and you come and you punch someone like 70 years old, like an old man, this shows your heart."

Bottom line, Khabib says he doesn't understand why someone close to McGregor doesn't speak up ... and tell him he's out of line.

"When you have parents and you have kids, how can you show yourself like this? I don’t understand why [Conor's] close people don’t tell him, ‘Hey, what’s going on?’”