Acu tetigisti.
You have hit the nail on the head. (Lit., you have touched it with a needle.) Plautus (Rudens, v 2, 19) says, “Rem acu tetigisti;” and Cicero (Pro Milõne, 24) has “Vulnus acu punctum,” evidently referring to a surgeon’s probe.
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.