From The Empire Strikes Back:
- Leia: I thought you knew this person.
- Chewbacca: [Chewie barks something to Han]
- Han Solo: Well, that was a long time ago, I'm sure he's forgotten about that.
It is it ever explained what Chewbacca was referring to? Is it shown in any EU works? Did they explain it in the movie and I just missed it?
Han won the Millennium Falcon from Lando in a game of Sabacc.
Wookieepedia says:
In addition to several thousands of credits, Calrissian bet any one ship on his lot on Nar Shaddaa. Calrissian almost won the tournament, only being one card away from holding an Idiot's Array. His final hand included The Idiot, The Two of Staves and The Seven of Flasks. Solo won the tournament with The Queen of Air and Darkness, The Five of Coins, The Six of Staves, and The Master of Coins - Pure Sabacc.
Solo won the 20,000 credits in the sabacc pot, plus roughly the same amount again he had won throughout the four days. When Calrissian asked Solo to choose any ship on his lot, Han picked the Millennium Falcon without hesitation. Calrissian protested, stating that it was his personal ship and not part of the deal, but Solo insisted that it was technically "on his lot" and therefore a valid choice. Calrissian finally conceded, and the Falcon was now the property of Han Solo.
This event was covered in the 3rd book of the Han Solo trilogy Rebel Dawn.
It is alluded to in Empire Strikes Back:
Lando: What are you doing here?
Han Solo: Ah, repairs. I was hoping you could help me out.
Lando: What have you done to my ship?
Han Solo: YOUR ship? Hey, remember you lost her to me, fair and square.
In Dark Apprentice they have a rematch that Lando wins, but to impress Mara Jade he gives the Falcon back to Han. Wookieepedia says:
Soon after, Lando disputed the validity of the sabacc game in which Han had originally won the Falcon from him, leading to a series of rematches between them. In the first, Lando won the ship back from Han and started an attempt to upgrade and rationalize the Falcon's computer systems. It was not quite finished when in a second game, Han won the ship back. In a final game—which both men agreed would be their last rematch over the Falcon—Lando won the ship back again, but in a public attempt to impress Mara Jade, gave the Falcon back to Han.
The Millennium Falcon originally belonged to Lando, Han won the ship in a card game (Sabacc). It's never made clear if Han cheated or won it legitimately, but it's insinuated that Lando wasn't too happy about it, and might be holding a grudge from it.
During the final round of the Cloud City Sabacc Tournament, Calrissian threw in a marker for "any ship on his lot" (at the time, he was the owner of a used spaceship lot on Nar Shaddaa, where the Falcon was parked). Solo, the other contestant remaining in the tournament, accepted the marker, knowing that Calrissian had taken a cruise ship to the tournament and left the Falcon behind. When Solo won the game, he claimed the Falcon as his own on the technicality which had been overlooked by his unlucky opponent.
From the Legends perspective, I see it as backlash from events depicted in Rebel Dawn, the depiction of Bria's (Han's love interest) actions towards Lando, confiscating his property in the name of the Rebel Alliance. Defaced and being branded a traitor back on Nar Shaddaa, Lando refuses to believe Han was not involved in the swindle, and punches his former friend in the jaw.
I'll answer with some new Disney Canon from Solo:
Han won the Millenium Falcon from Lando by stopping him from cheating at the end of the movie Solo. The way he did it was with the same kind of weird hug greeting that Lando does to him in ESB.
Here's the relevant info from the script (emphasis mine):
Lando: (ALL EXCLAIMING) Ooh. (CHUCKLES) You really have it bad for the Falcon, don't you? It's mutual, trust me. She belongs with me. (LANDO CHUCKLES) You got everything you need there, pal?
Solo: (CHUCKLES) (ALL CHEERING) Not bad. Not bad. You know, if you'd only had this little green sylop, you would've beaten this.
As you can see, he stole the card that Lando uses to cheat at Sabaac and therefore wins the game. Solo then says he played 'fair and square' but I kinda doubt it, and Lando clearly thinks he's been tricked out of winning, despite trying to cheat Solo out of winning.
So in a weird turn of events, Disney didn't change this backstory very much at all.
Lando hated Han because on their last mission together Han convinced a huge group of smugglers to help raid Ylesia with the Rebel Alliance in exchange for treasure. Han was dating Bria Tharen the Rebel Alliance commander and she didn't tell them they were planning on double-crossing the smugglers because they needed the treasure to pay for Alliance stuff. When the double-cross went down Han went with his girlfriend instead of standing with the smugglers, betraying them all.
