See definition of 'curlbro'.
Somebody who only works out his arms and neglects everything else. Unproportional/Unaesthetic fitness people whose main goal is to impress others with their big arms. These people are in full-on bromode.
John: You know what I like about all those bromodes in our gym? The power-rack is usually empty. No waiting time when you want to do squats!
Jim: Yeah man, gotta love them curlbros.
5 minutes later
Jim: DUDE WHAT THE FUCK I CAN'T DO MY SQUATS BECAUSE ONE OF THEM BROMODE ASSHOLES IS USING THE SQUAT RACK FOR HIS INCLINE CURLS.
John: I CAN'T DO BENCHPRESS BECAUSE THERE'S A FUCKING CURLBRO SITTING ON THE FUCKING BENCH DOING CONCENTRATION CURLS, WAT THE FUK MAN.