A wangjja (왕자, 王字, pronounced 왕짜) is a "six-pack", the visible set of muscles on a person's abdomen. The name comes from the name of the hanja, the first not to signify king but rather the shape of the muscles:

王

and the second one, 字, means written character. The literal meaning is a "王-shaped mark". Note that this word is pronounced differently from the other word written 왕자 (王子), a prince. This word is pronounced "왕짜," while "prince" is pronounced "왕자."