For the life of you
Definition 1 of 1
Pro Tip 1/3Emphasis on Frustration
Often shows frustration or impatience over not being able to do something. 
I can't, for the life of me, figure out this puzzle.
Pro Tip 2/3Used for Emphasis
Adds emotional emphasis, suggesting you've tried very hard or care deeply about the effort. 
She couldn't understand the poem for the life of her, even after rereading it several times.
Pro Tip 3/3Not Literal
It's an idiom, so don't take it literally; it doesn't involve actual life or death. 
He couldn't, for the life of him, recall where he had seen her before.