Morning wood

Morning wood or piss-hard is an erection on awakening.

Males sometimes become erect during sleep, for reasons which are reasonably well understood. Erotic dreams, with or without nocturnal coming, are not the only reason. Stimulation from friction of bedclothing, periodic autonomic system arousal during sleep, etc have all been suggested as causes. A full bladder can create morning wood, often by pressure exerted on the prostate by the bladder.

In a D/s context, a submissive may be required to cause an erection during the night so as to provide an opportunity for the Master's use, but this tends to be a golden showers scene. The D can come and piss in the mouth simultaneously.