In the dark they dig through houses. In ancient times, burglary commonly took this form. Windows were few, and high up in the walls; doors were strongly fastened with bolts and bars. But the walls, being of clay, or rubble, or sun-dried brick, were weak and easily penetrable.
This was especially the ease with party walls; and if burglars entered an unoccupied house, nothing was easier than to break through the slight partition which separated it from the house next door. The Greek word for "burglar" is τοιχώρυχος'" he who digs through a wall."
Which they had marked for themselves in the daytime; rather, they shut themselves up in the daytime; literally, they seal themselves up; the meaning being that they carefully keep themselves close. Professor Lee, however, defends the Authorized Version.
They know not the light; i.e. they avoid it, keep away from it, will have nothing to do with it.