couldn't (do something) to save (one's) life
slang Cannot do something well or at all. Ugh, this art assignment is so annoying—I couldn't draw an apple to save my life. Rachel is so selfish! She couldn't consider another person's feelings to save her life.
to save (one's) life
Regardless of one's effort, passion, or desire. Ugh, this art assignment is so annoying—I can't draw an apple to save my life. Rachel is so selfish! It's like she couldn't consider another person's feelings to save her life.
to save your life
used in various expressions, especially can't (or couldn't ) do something to save your life , to indicate that the person in question is very incompetent at doing something. The first recorded use of this expression is by Anthony Trollope in The Kellys and O'Kellys ( 1848 ): ‘If it was to save my life and theirs, I can't get up small talk for the rector and his curate’.