Regularly hitting the sheets with someone youare not dating OR casually moving in with someone you regularly hit the sheets with.
Living in sin.
Donna spends all her time over at Jim's house, they have been shacking up together ever since they hooked up on Fat Tuesday.
Litteraly: Without Paper, experiencing a bowel movement when no toilet paper is available.
My roommates never buy toilet paper and I had to WOP Drop this morning.