Colin Farrell's Shirtless Workout in Dad Shorts

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.