Brewer's: 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 Milone, 24) has “Vulnus acu punctum,” evidently referring to a surgeon's probe.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894