During the filming for season two, the 39-year-old hunk was challenged on set to "the slap game" - which sees one player slapping the other's hands until they miss and swap over - by David Benioff, and it didn't go too well for the fantasy show's co-creator.
Speaking on HBO's 'Backstories' web series, David said: "Jason was one of our favourite people ever on the show, and very strong. And that's not TV makeup - in real life, he's just a big, strong dude.
"So, I'm looking at Momoa and Momoa's talking about beating someone at the slap game. I was like, 'I'll take you at the slap game.' I was like, 'I'm going to challenge the Khal, because if I beat the Khal, I am the Khal.'"
Jason returned to the role as towering Khal Drogo after dying during season one. He returned for a dream sequence with Emilia Clarke - who played his former on-screen wife Daenerys Targaryen - and the behind-the-scenes battle gripped the cast and grew.
David laughed: "I looked in Momoa's eyes, I saw no mercy there. But I wouldn't quit, because, you know, I had my pride. I had my dumb pride."
His hands "doubled in size" by the next morning, and when he got back home to his wife Amanda Peet in Los Angeles, she sent him straight to the hospital.
He added: "Her diagnosis was that Jason Momoa had squished my hands ... So, it just goes to show, don't challenge the Khal."
Meanwhile, Jason recently said he was "very sad" after Emilia nearly died "numerous times" when she suffered two life-threatening brain aneurysms while filming the show.
He revealed: "I'm very sad, because we almost lost her numerous times. I love her to bits and she's here and she's going to do great things with it and teach the world."
Where to watch "Game of Thrones"