ad unguem

Etymology
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Adverb

 * 1) ad unguem

Etymology
(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.

Adverb

 * 1) to a hair, to a nicety; precisely, exactly; to an exacting standard