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1. To brighten something in color. I think some new highlights will lighten up your hair and really refresh your look.
2. To act less seriously or sternly. In this usage, "lighten up" is a set phrase. A: "I think he definitely needs to be grounded." B: "Oh, lighten up—it's not like you never made stupid mistakes as a kid."
3. To cause someone or something to become less serious or stern. It's a funeral—nothing you do will lighten up the mood.
lighten something up
to make something lighter or brighter. Some white paint will lighten this room up a lot. The sunlight came in and lightened up the kitchen.
lighten up (on someone or something)
to be less rough and demanding or rude with someone or something. Please lighten up on her. You are being very cruel. You are too harsh. Lighten up.
to become lighter or brighter. (See also lighten up (on someone or something).) We applied a new coat of white paint to the walls, and the room lightened up considerably. The sky is beginning to lighten up a little.
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.
1. To make something appear lighter in shade: The artist lightened up the paint by mixing in some white. Those mirrors really lighten the room up.
2. To become lighter in shade: These pants are too dark now, but they will lighten up after they've been washed a lot.
3. To make something more pleasant and less serious or depressing: The jokes lightened up the tone of the meeting. I didn't mean to be disrespectful—I was just trying to lighten things up.
4. To take matters less seriously: Everything will work out fine, so stop worrying and lighten up.
Don’t be so serious, relax. Originally, this slangy imperative, dating from the 1940s, meant to calm down, but in succeeding decades it took on its present meaning. Tracy Kidder had it in Among Schoolchildren (1989): “Me and my precious schedules . . . I’ve got to lighten up. Chill out.”