This is the adverb cousin of expressionless: you do something expressionlessly when your face or voice stays blank while you do it. She answered expressionlessly. He stared expressionlessly at the floor. Same idea — no readable feeling — only now it tells you how the action happened, not what the face is like as a quality. It is not an everyday spoken word; in speech people more often say with no expression or without showing anything. You will meet it in stories, reports, and careful writing.

面無表情地