The act of ejaculating onto/into your partners pilliowcase. Then when your partner wakes up the next morning your invisible ink has dried, leaving a white film on the pillowcase.
The reason its the "Seattle" Sleeper is because the city always gets a light dusting of snow that you can only really see in the morning.
I woke up and my partner had left a Seattle Sleeper on my pillow, it took a little more effort than usual to pull my face away.
When a males ejaculates on their partner's face while they are asleep.
My girl wouldn't give it up last night so I jizzed on her face when she was asleep. Man alive, she hates a Seattle Sleeper.