When you wake up from an extreme night of drinking with an incredible erection. The muscles in your penis contract so that you don't pee on yourself in your sleep.
Usually beneficial if you have a female with you the next morning, as you can go for a long time since your penis is trying to not let anything release.
Usually only relieved by urinating (which is hard to do in that condition)
Man...I woke up this morning with Whiskey Dick after all that beer last night...I could have used it to go out and drive nails into the sidewalk!