By TMZ Sports

Former heavyweight champion Shannon Briggs is calling out the heavyweight champ -- saying Deontay Wilder is a lying "COWARD" who's smearing his good name ... because he's secretly scared out of his mind.

Briggs appeared on "TMZ Sports" on FS1 (Monday @ 9PM PT, 12AM ET) -- to respond to Wilder's insinuation that Briggs couldn't pass a drug test if the two agreed to fight on January 16th at the Barclays Center in Brooklyn. 

"That's what cowards say. He's a coward. I just had eight fights in a row and passed every drug test. Where are you coming from with this? Why? That's coming from inner fear. That's his heart saying 'make up something.' That's his people too," Briggs said.

"They called me and asked if I would fight Deontay Wilder and I said 'okay, I want $2 million dollars'. And they said they would call me back. They said they would give me $500,000 and I said 'okay, lets do it' and they said they would call me back. They called back and said $400,000 and I said 'no problem'. It aint about the money. I would fight him for free - winner take all in Brooklyn. I'm so confident that I would knock him out. He wouldn't even go past one round."

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