Cohabit outside marriage, as in Bill and Anne lived in sin for years before they got married. This term, dating from the early 1800s, is mostly used in a jocular fashion today, when customs and views are more liberal in this regard. Also see live together.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.