It's akin to the more widespread 'morning wood', but instead of being a function of a typical male, it is instead a daily reminder to necrophiliacs. It does involve an erection upon regaining consciousness, but instead of being caused by lustful porn stars, it's caused by sadness that your lover last night is now six feet under, and you are depressed, but more so you are excited at the thought of having them once. Hence the title 'mourning wood'
Doug: Hey man, that party we had at the cemetery last night was a blast.
Mike: Yeah, i found a 40-something blond amongst the oldies. I definitely had some mourning wood when i woke up.