The Collaborative International Dictionary of English v.0.48:

Shack up \Shack" up`\, v. i.
   1. to live together in a sexual relationship, without being
      legally married. [Slang, U. S.]
      [PJC]

   2. to live in a cabin, shack, or other crude dwelling.
      [Slang, U. S.]
      [PJC]

WordNet (r) 3.0 (2006):

shack up
    v 1: share living quarters; usually said of people who are not
         married and live together as a couple [syn: cohabit,
         live together, shack up]