1. To make less heavy.
2. To lessen the oppressiveness, trouble, or severity of.
3. To relieve of cares or worries; gladden.
1. To become less in weight.
2. To become less oppressive, troublesome, or severe.
3. To become cheerful.
lighten up (Informal)
To take matters less seriously: Everything will work out fine, so stop worrying and lighten up.