Idioms and Phrases with lighten up
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.