J.K. Simmons: INSANELY JACKED ... For New 'Justice League' Flick

Well, now we know what the J in J.K. Simmons stands for -- JACKED.

Check out the 61-year-old beasting out in an L.A. gym -- gearing up for his role in the upcoming 'Justice League' movie.

No, he's not playing Bane ... or Batman ... he's an insanely muscle-bound Commissioner Gordon!

Simmons has been training with retired Marine Aaron Williamson -- who reportedly got Zac Efron into shape ... and who's been known to push weights with The Rock.