Become discouraged, as in The rescuers worked hard for the first few hours, but then they lost heart. This term uses heart in the sense of “courage” or “spirit.” [Mid-1800s] Also see lose one's heart to.
Words nearby lose heart
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.