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For the first time ever -- that we can find -- Quinton "Rampage" Jackson has revealed how he got his legendary nickname ... and it all stems from a violent 8-bit gorilla.

The former UFC champ says his cousin coined his famous moniker after playing the arcade classic from the '80s ... "Rampage."

For the uninitiated, Rampage featured a couple of smashaholic monsters pummeling their way through cities by plowing their fists through skyscrapers ... it was awesome.