Mirthless means there is no real laugh in it — no warm amusement, even if the face is doing the shape of one. People use it most about a smile or a laugh that is empty: the mouth moves, but the joy is not there. A mirthless laugh can sound bitter, tired, or cold, as if the person is making the sound because that is what you do, not because anything is funny. It is a quiet, slightly dark word, more at home in books than in chat. Though actually, mirthless does not mean silent. The sound can still be there. What is missing is the mirth.