A person who has recently began lifting weights, acquiring minimal to some muscle definition, and feels the need to show off by wearing tight-fitting short sleeve shirts and tank tops - even on cold days.
Similar to new money.
"Damn, it's gotta be like a 40 degree day out here."
"No shit, and that mother fucker over there is wearin' nothin' but a wife beater. He's gotta be new muscle."