1. To be living or sharing accommodations with someone while having a casual sexual relationship with them. I haven't seen my sister in weeks. I hear she's been shacked up with some guy from Oklahoma these days. I was shacked up for a while with a woman I met downtown, but I've been on my own for the last couple of months.

2. To temporarily share accommodations with someone. A: "Do you have a place to stay while you're in town for the wedding?" B: "Yeah, I'm shacked up with my brother and his fiancée for the weekend."