noun
workaholic; work fiend; devil for work
Describes someone who is extremely dedicated to work, often to an obsessive degree. The word 鬼 (oni, demon) is used metaphorically to emphasize intensity and single-mindedness.
彼は本当に仕事の鬼で、休日もほとんど働いている。
He's a real workaholic; he works even on most of his days off.
あの課長は仕事の鬼と呼ばれている。
That section chief is called a work fiend.