Sorry, LSU ... Billy Bob Thornton ain't buyin that "horseplay" argument -- saying the Leonard Fournette video sure looks like a fight to him.
Thornton joined the guys on the "TMZ Sports" TV show (airs tonight on FS1) and said he was pretty confident that the footage in the video was 100% real.
We broke the story ... LSU officials claim Fournette and teammate Garrett Brumfield were just slap boxing ... and hugged it out after the video ended.
FYI -- You're not gonna wanna miss BBT on the show Wednesday (airs 9 PM PT) ... dude is awesome as hell and really knows his sports.
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