Lighten up Definition 1 of 3Pro Tip 1/3Social Context Used among friends or in casual settings, not in formal situations. At the party, Mike told me to lighten up and enjoy myself. Pro Tip 2/3Mood Shift Implies a change from being tense or serious to more cheerful. When she said to lighten up, I realized I was taking the game too seriously. Pro Tip 3/3Emotional Subtext Can be used to show empathy or concern for someone's well-being. You've been so stressed lately. You should lighten up and treat yourself.