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.
no saber distinguir el culo del codo, no saber nada
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.
Es tan incompetente que no sabe distinguir el culo del codo.
•
Don't ask him for help; he doesn't know his ass from his elbow when it comes to computers.
No le pidas ayuda; no sabe distinguir el culo del codo cuando se trata de ordenadores.