The current free agent -- who recently got cut by the Lions -- was just minding his business and sharing his 2 cents about his old teammate, Todd Gurley, on Sunday ... when one guy decided to bring up his weight.

"At least you got a good view from your couch fat boy"

Now ... there are several ways to go about responding to Twitter trolls, but if you got money like C.J., the obvious choice is to bring up the bank account.

"Being paid to sit here my man. I believe that’s the difference between me on the couch and you."

Ouch.

But, the troll wasn't going down without a fight, hitting C.J. back with "Just be nice if you would spend as much time exercising as you do on Twitter , good to see your proud of getting paid to sit on the couch"

C.J. wasn't having any of that talk ... bringing everything back to perspective to land one final blow.

"This can go both ways right? ' I need to get off twitter to hit the gym' however, your trolling another grown man instead of trying to make sure ya life is straight. Lol you have officially become a clown."