The word curlbro consists of the two words curl (for biceps-curl, a popular isolation exercise in hypertrophy training) and bro (brother, but here: fake tan, borderline-retarded, gel-in-hair, unintelligent person).
A curlbro enjoys lifting weights in his free time. The focus of his workout regimen is on 'getting big arms' - because that's the only part of your body that usually sticks out when curlbros are 'in da club, partying it up wid da chixx'.
A curlbro usually doesn't know anything about fitness and his goal isn't to develop an aesthetically pleasing body but rather just grow big 'disco muscles, bro!' to impress 'da chixx, brah!'.
John: Hey man, did you see that new guy at the gym? He comes in three days a week and all he ever does is biceps, triceps and forearm isolation exercises. The fuck, man? His arms are bigger than his legs. It looks completely out of proportion and retarded.
Jim: Yeah man, total curlbro. He's also looking kinda orange, I guess he's got the fake-tan thing going as well.
John: Well, yeah. That's kind of obligatory. He's also wearing shades during workout and fixes his hair throughout his curlbro routine.
Broseph: YO GUYZ CAN YOU SPOT ME I WANNA WORK OUT MY BICEPS UNTIL FAILURE LOL GOTTA GET DEM VEINS GOIN 4 DA CHIX IN DA CLUB TONITE?!??!
Jim, John: Excuse me, I have an actual workout to continue.
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.