Floyd Mayweather Sr. is coming out swingin' at Justin Bieber -- saying the pop star doesn't know a DAMN THING about boxing if he thinks Conor's gonna go the distance.
Remember, Bieber -- Floyd Jr.'s longtime friend -- recently shot down the idea that Mayweather would K.O. McGregor ... and that ain't sittin' too well with Floyd Sr.
"Justin Bieber ... he don't even know boxing," Floyd's dad told us at LAX ... "He knows Floyd. But he don't know Floyd like I know Floyd."
Floyd Sr. also went in on Vegas gamblers putting their money on Conor -- saying they're crazy ... and a bet on Floyd is like taking free money!
Papa Mayweather also says he thinks his kid should STOP FIGHTING after Aug. 26 -- because if he keeps taking on challengers, someone is eventually gonna knock his ass out!