E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
I have it at my fingers ends. I am quite familiar with it and can do it readily. It is a Latin proverb (Scre tanquam ungues digtosq.), where the allusion is to the statuary, who knows every item of his subject by the touch. (See UNGUEM.)
Costard: Go to; thou hast it ad dunghill, at the fingers ends, as they say.
Holofernes: O, I smell false Latin; dunghill for unguem.Shakespeare: Loves Labours Lost, v. 1.