Specifically, occasions when one goes to clean their clothes with someone else and gets more than the wash done.
In general can be used for any type of hook up.
Gabe: "Yea man, she's down to do The Laundry!"
Maddy: "Do you know where Grace went last nightt?"
Krystal: "She ended up at Michael's"
Maddy: "Yeaah, I hope she got The Laundry done!"
Mary went to the frat quad to do The Laundry, and came back wearing clothes that were particularly clean, if you know what I mean.