The process illustrated:
Facebook message to housemates about machine man telling us the house is haunted -->
We have a haunted house -->
We must do something - get Juliette and the incense! -->
Our house isn't haunted, our WASHING machine is, which means, logically, OUR UNDERWEAR IS NOW HAUNTED! -->
Our underwear is possessed, so we're possessed, so we possess a haunted house ohnoes! -->
Must protect ourselves from the haunted underwear -->
"I'll possess your underwear. ;)" -->
"ARE YOU COMING ON TO ME?!" -->
"No, I'm coming on to anywhere." -->
(What would possess you to do that? Well, me, possessing your underwear.) "That is the best phrase ever."
And thus was born the phrase "I'll possess your underwear" out of a few screws loose in a laundry machine.
(Personally, I don't think the machine's the only thing with a few screws loose.)