Shannon Briggs -- Deontay Wilder's a Coward ... I'll Fight You For Free!

Boxing superstar Shannon Briggs is calling out the heavyweight champ -- saying Deontay Wilder is a lying "COWARD" who's smearing his good name ... 'cause 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 in January.

Briggs RIPPED Wilder apart -- and then called B.S. on Wilder's other claim that he's asking for too much money ... telling us he'll fight Wilder FOR FREE!!!!

Ya gotta watch the clip ... and the entire segment on FS1 tonight ... dude's PISSED and guaranteeing he'd K.O. Wilder in one round if they do step into the ring together next month.