Conor McGregor has ignited a rivalry with Islam Makhachev by replying to tweeted posted by the rising UFC star in 2015.
Makhachev has set his sights on a lightweight title shot in the coming months - the belt once held by McGregor.
The Russian won nine fights in succession in the UFC, including a dominant first-round finish of Dan Hooker last weekend.
He could now be granted a shot at the winner of champion Charles Oliveira's clash with Dustin Poirier next month.
But McGregor, who is currently recovering from a broken leg, appears to have taken issue with Makhachev's rise.
He dug out a tweet from 2015 before McGregor fought Chad Mendes for the interim featherweight title, captioning a picture of their weigh-in with: "I can't imagine after all his talk, Conor loses."
McGregor, who went on to stop Mendes before knocking out Jose Aldo to become the 145lb champion five months later, has belatedly replied: "Can you not, you inbred? What about the one after that then? You are nobody without me, don't forget. Beg me.
"If I leave s*** on the bus on that bus, he a nobody like you, ahahahaha. He still is, kid. A nobody at the big bank. S*** the bus and now he lives off your work."
McGregor's reference to the bus relates to a heated incident in 2018 when the Irishman threw a trolley at a bus ferrying UFC fighters, including bitter rival Khabib Nurmagomedov, to their hotel.
McGregor was later charged and pleaded guilty to disorderly conduct in order to avoid a jail term.
The Dubliner was then choked out by Nurmagomedov in the fourth round of their grudge fight later that year. The Dagestan native retired from the UFC last year and now coaches a number of fighters including Makhachev.
Following McGregor's tweet, which was later deleted, a fan asked him to prove himself by taking on Makhachev when he returns from injury.
But McGregor said: "No. His cousin is his dad."
Should Conor McGregor take on Islam Makhachev when he returns? Let us know in the comments section
The 33-year-old recently returned to his gym in Ireland where he has stepped up his rehabilitation.
He filmed himself hitting pads earlier this week and promised to deliver the "greatest comeback in sports history" next year.
"First time back on the pads! What a buzz! Let’s go team. Like my good friend Arnold Schwarzenegger says, 'I’ll be back, b******!' - back with a vengeance!
"Thank you to all my doctors and my team for getting me this far so soon after surgery! There is work to be done but doing work is what I’m known for. Strap yourselves in for the greatest comeback in sports history!
"[In] two more months I am able to kick, grapple and run again. Ecstatic and grateful at my position currently.
"Watch the work get put in now. Eight weeks left this calendar year, perfect to get a head start on 22’. You think a broke bone stop me?"
