Provide hospitality for visitors, as in They loved company and kept open house virtually all summer long. This usage differs from the plain open house, which refers to a particular occasion or period when a home or institution is open to visitors or prospective buyers. [First half of 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.