Remember ... Floyd gushed over McGregor to TMZ Sports on Monday, calling the Irish fighter a "warrior" and even inviting him to train for the Khabib fight at the Mayweather Boxing Club in Vegas.
But Conor clearly wants no part of that ... declining Floyd's offer on Tuesday with a fat middle finger.
"F*ck the Mayweathers, except Senior and Roger," Conor said.
"There is no peace here, kid. Step up or step down."
Floyd responded to Conor's tweet later Tuesday night by posting a pic from the fight ... saying "Easiest 9 figures I ever made!"
Obviously ... there's still bad blood on Conor's end from when the two fought in the boxing ring last August. McGregor lost ... and then had to deal with Floyd's trash talk over their Forbes List rankings (Floyd #1, Conor #4) a few months later.
Seems like there's really only one way to settle this for good ... REMATCH!!!