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.