Become or cause to become less serious or gloomy, and more cheerful. For example, Lighten up, Sam—it'll turn out all right. This slangy expression transfers reducing a physical weight to a change of mood or attitude.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.