can't do something to save your life
can't (do something) to save (one's) life
slang Cannot do something well or at all. 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 can't consider another person's feelings to save her life.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
can't do something to save your lifeSPOKEN
If you say that you can't do something to save your life, you mean that you cannot do it at all. I'm never nervous at exams but I can't study to save my life. He can't sing to save his life but he's a good guitarist.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
can’t do something to ˌsave your ˈlife(informal) cannot do something at all or can only do something very badly: He can’t cook to save his life.
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