Halfway up or down, as in The church bells tolled off and on all day and the flags were at half-mast. This term refers to placing a flag halfway up a ship's mast or flagpole, a practice used as a mark of respect for a person who has died or, at sea, as a distress signal. Occasionally the term is transferred to other objects, as in Tom's pants were at half-mast as he raced around the playground, or The puppy's tail was at half-mast. [First half of 1600s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.