Tito Ortiz is clapping back at the haters killin' him for steppin' back in the cage against Chuck Liddell ... issuing an invitation -- come to my gym and get your ass busted in real life.
TMZ Sports talked to "The Huntington Beach Bad Boy" on Tuesday -- days before battling "The Iceman" for the 3rd time -- and asked him if he had a response to people saying he's too old and washed up to fight.
That's when 43-year-old Ortiz went off ...
"Those people are the guys who sit on the couch and they really don't do nothing," Ortiz told us ... "We call it computer tough guy stuff. Get in the gym and do something and see how hard it is to train."
Matter of fact ... Tito wants to bring in a crap-talkin' fan and see if he can hang in the gym.
"Come join me for a full day of training in my 3 sessions, and try to get through it, and see how slow I am. Or how easy I go. Man, I go and I battle like a beast when I train. I'm vicious."
"When people say I'm old and they feel sorry for us. Don't feel sorry for me. I'm on point. Maybe feel sorry for Chuck for a little bit. I feel sorry for him. That's why I took the fight."