to be less serious about something.

People are usually relieved when they're given a chance to lighten up.

Usage notes: often used as an order:

When she complained that these people were being treated badly, he told her, “Lighten up.”

Related vocabulary: get a life

• have your back against the wall
• have your back to the wall
• have your beady eye on
• have your beady eye on someone
• have your beady eye on something
• have your cake and eat it
• have your cake and eat it too
• have your day in court
• have your ducks in a row
• have your ear to the ground
• have your end away
• have your eye on
• have your eye on someone
• have your eye on something
• have your feet on the ground
• have your finger on the button
• have your finger on the pulse
• have your fingers in the till
• have your guts for garters
• have your hand in the till
• have your hands full
• have your head in the clouds
• have your head screwed on
• have your head screwed on right
• have your head screwed on the right way