Be excited, anxious, or nervous. For example, The election was so close that I held my breath until the final results were in, or I'm holding my breath until every-one's been heard from. This expression alludes to the interruption of normal breathing; the literal usage dating from the early 1700s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.