An unexpected sleepover at someone else's place. Sex may or may not happen. The term shacking does not apply to staying the
night expectedly, when you have your own change of clothes and a toothbrush. A shack often involves borrowing or stealing the
guy's clothes for the walk of
shame the morning after, thus the term "shacker outfit."
"
Yeah I shacked at his place last
night after Paddy's."
"Nicole didn't come home last
night, she shacked at Ryan's."