It's often used to describe the feeling of envy or possessiveness that can arise when someone else has something you want, or when you feel threatened by someone else's success.
The term likely comes from the Green Man, a figure in folklore and mythology often depicted with green skin and foliage, representing nature and the power of growth. In this context, the "green monster" represents the overwhelming and uncontrollable nature of jealousy.
Here are some examples of how the expression is used:
* "The green monster of jealousy consumed him when he saw his ex-girlfriend with another man."
* "Don't let the green monster ruin your happiness. Be happy for your friend's success."
* "The green monster of envy can be a powerful emotion, but it's important to control it."