the real McCoy
informal something that is neither imitation nor substitute; the genuine article
We thought the Gucci watch was fake but it turned out to be the real McCoy
[an alteration of Mackay in the phrase the real Mackay the true chief of the Mackay clan, a position that was often disputed. The distillers G Mackay promoted their whisky as ‘the real Mackay’ in the 1870s, and R L Stevenson used the expression in a letter of 1883.]