Colin Farrell doesn't need a fancy gym with top-of-the-line equipment or the fittest exercise clothes from Under Armour ... just a couple of dumbbells and a pair of khaki shorts work just fine.
The "Dumbo" star pumped it up at a private gym in L.A. Thursday ... despite possibly forgetting his gym bag containing his t-shirt, athletic shorts and tennis shoes.
Instead, Colin went shirtless and showed off his rock-hard dad bod along with his new mohawk-style haircut ... and looked pretty serious about getting his reps in.
We're told Farrell's currently on a break from shooting his new film, "Voyagers," in Romania ... and he's using his free time to stay fit.
Along with using the free weights, Colin hopped on the stationary bike for some cardio. Unclear if he used any of the other machines ... but hopefully he wiped them down after he was done ... or not.
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