Not be able to do something to save your life
Definition 1 of 1
Pro Tip 1/3Extreme Inability
The phrase emphasizes a dramatic lack of skill, indicating that the person is unable to do something no matter how hard they try. 
I tried to fix the sink, but I couldn't do it to save my life.
Pro Tip 2/3Hyperbolic Humor
Often used humorously to exaggerate personal shortcomings in a self-deprecating way. 
Even with a recipe, I can't bake cookies to save my life.
Pro Tip 3/3Idiomatic Context
Understand that this is an idiomatic expression, not to be taken literally. It's not about actual life-threatening situations. 
He's so clumsy, he couldn't catch a ball to save his life.