Simple ... Tony says it's a combination of crazy genetics and mental fortitude.

The UFC star opened up about his jacked up arm on social media late Monday -- saying it's not as bad as it looked.

"My Arm Is Aright," Ferguson posted on IG ... "The Armbar Was Really Tight."

"The Thought Of My Son ArmandAnthony’s Ability To Be Double Jointed In His Elbow During That Sequence & My Mental Capacity To Endure Pain Got Me Through It. # Breathe # MentalTUFness."

There have been rumblings that Ferguson could consider wrapping up his MMA career after 2 bad losses in a row ... but T-Ferg says he ain't going nowhere.

"We Are Far From Being Retired," Tony said ... while dropping a middle finger emoji to his haters.

"I Can’t Wait Til We Compete In Front Of A Big Loud Crowd Again, It’s Not The Same Without You All."

In the meantime, Ferguson says he's working on healing up his arm -- with a focus on increased blood circulation.

He even underwent some cupping therapy to help out.