Green’s Dictionary of Slang

a derog. term for a Scot. Presbyterian minister, as opposed to an Anglican or Roman Catholic.

c.1661		J. Taylor Course of Sermons in Works (1826) II 524: To prefer the private minister before the public, the presbyter before a bishop, [...] and Mas John before the patriarch of Jerusalem .
1700		T. Brown Amusements Serious and Comical in Works (1744) III 101: What he calls a prayer [...] has furnish’d many a zealous Scots Miss John with an exercise for his lungs.
1772		G. Stevens ‘The Sweethearts’ in Songs Comic and Satyrical 244: The next a Mess John, of rank Methodist Taint, / Who thought like a sinner, but look’d like a saint.
1785		Poems in the Buchan Dialect ii 42: This breeds ill wills, ye ken fu’ aft, In the black coat, Till poor mass-john and the priest-craft Goes ti’ the pot [F&H].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Mess John, a Scotch Presbyterian teacher or parson.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].