Be six feet under
Definition 1 of 1
Pro Tip 1/3Informal Idiom
The phrase is casual and often used in informal settings or among friends. 
I joked that if I ate any more cake, I'd be six feet under by thirty.
Pro Tip 2/3Mortality Reminder
The phrase reminds us of human mortality and is sometimes used humorously or ironically. 
I'll finally be six feet under before they finish that road construction.
Pro Tip 3/3Sombre Topic
It refers to death, which can be a sensitive topic, so use it with care. 
He wants to travel the world before he's six feet under.