E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 Milne, 24) has Vulnus acu punctum, evidently referring to a surgeons probe.