1The Church of Scotland; = "kirk".
2 informal, rare Whisky. Chiefly in "a drop of the Auld Kirk".
Probably partly with humorous allusion to the supposed fondness for whisky shown by ministers of the Church of Scotland (whereas ministers of dissenting Churches were better known for their temperance), but perhaps also partly with reference to the fact that the stipends of ministers of the Church of Scotland depended largely on the price of barley, from which whisky is made.
Auld Kirk/ˌɔːld ˈkəːk/ /ˌəʊld ˈkəːk/
Late 18th century; earliest use found in The Public Advertiser. From auld, Scots variant of old + kirk.