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.
一窍不通, 笨手笨脚
to be very stupid or ignorant; to lack basic knowledge or common sense
:
•
He's so incompetent, he doesn't know his ass from his elbow.
他太无能了，简直一窍不通。
•
Don't ask him for help; he doesn't know his ass from his elbow when it comes to computers.
别向他求助；他对电脑简直一窍不通。