Charlie Sheen is going to be a GRANDPA!!!!!!!!

Here's a scary thought -- Charlie Sheen is going to be a grandpa!

The Anger Management star made the announcement on Letterman Monday night that his 28-year-old daughter, Cassandra Estevez, is pregnant. Charlie admitted that while "it's fabulous," being a grandpa is also "not a title I'm ready to adopt."

When Dave asked whether Charlie would be a "good grandfather," the 47-year-old actor said, "Yeah, because you can leave the kid!"

Cassandra is Charlie's daughter with his high school girlfriend Paula Speert