not know one's arse from one's elbow

idiom

British, informal + impolite
: to know nothing : to be stupid
Don't take his word for it—he doesn't know his arse from his elbow.

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“Not know one's arse from one's elbow.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/not%20know%20one%27s%20arse%20from%20one%27s%20elbow. Accessed 3 Mar. 2025.

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