When you lean over to wash your hands in a public restroom, and there is water on the counter left over from the pervious user, getting that on your shirt is called being attacked by the sinkmonster.
-"What's that on your shirt?"
-"That? Oh I leaned over to far and was attacked by the sinkmonster."
The act of masterbating in the middle of the night whilst your partner is sleeping. Then on the brink of ejactulation, you let out a horrify shriek while unloading on the their face.
"Dude, last night I totally pulled a monster's ink on my chick! She woke up screaming!"