Welp, that didn't take long -- Conor McGregor just fired back at 50 Cent for claiming black people have "superior genes" ... saying the rap star should pull out his old-man boobs to celebrate women's day.
"Get your tits out for the lads, ladies," McGregor said in a social media rant ... which he attached to a picture of a shirtless 50.
"I am made of granite. You need a bra."
Conor also told 50 to "f**k off" ... 'cause he's a 50-year-old fool (he's actually 42) who needs to stay in his lane.
Surprisingly, Conor also worked in a semi-compliment with his shots -- saying they still bump his old hits in Ireland ... and claiming he rocked out to one before UFC 205.
Doubt they'll break bread anytime soon ... but it's a start?