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.
Cite this Entry
“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 28 Nov. 2025.
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Merriam-Webster unabridged
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