For sale, as in These paintings and Oriental rugs must come under the hammer if we're to pay the mortgage. This expression alludes to the auctioneer's hammer, which is rapped to indicate a completed transaction. [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.