Definition of "unmirthful"

Through the chinks of my fingers, dolorously spread over my face, I see that he has sat down on the other side of the table, just opposite me, and that he is smiling in the same unmirthful, gently sarcastic way, as he was when he left me.

1874, Rhoda Broughton, Nancy‎

As she gazed, an unmirthful smile spread over her features, like sunshine that grows melancholy in some desolate spot.

1903, Philip P. Wells, Bible Stories and Religious Classics‎