1
general::
Phrase(s): shack up (with someone)
Inf. to sleep or live with someone temporarily in a sexual relationship. • They shacked up for over a year until her parents found out and stopped sending her money.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
1. Sleep together or live in sexual intimacy without being married. For example, They had been dating for two months and then decided to shack up. [Slang; first half of 1900s] 2. Stay or reside with, as in I'm shacking up with my cousin till I find a place of my own. [Slang; first half of 1900s]
American Heritage Idioms