not know your ass from your elbow
US /nɑt noʊ jʊər æs frəm jʊər ˈɛl.boʊ/
UK /nɑt noʊ jʊər æs frəm jʊər ˈɛl.boʊ/
1.
to be very stupid or ignorant; to lack basic knowledge or common sense
:
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He's so incompetent, he doesn't know his ass from his elbow.
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Don't ask him for help; he doesn't know his ass from his elbow when it comes to computers.