to a hair, to a nicety; precisely, exactly; to an exacting standard
köken: Calque of Ancient Greek εἰς ὄνῠχᾰ (eis ónukha, literally “to a nail”): an expression borrowed from sculptors, who, in modelling, give the finishing touch with the fingernail, or from joiners, who test the accuracy of joints in wood by the fingernail.