John Knox, n. — Green’s Dictionary of Slang

John Knox n.

[rhy. sl. = pox n.¹ (3); ult. Scottish Presbyterian clergyman John Knox (c.1510–72)]

(Scot.) venereal disease.

1973		(con. mid-1960s) J. Patrick Glasgow Gang Observed 117: A dose of the John Knox.
2018		I. Welsh Dead Man’s Trousers [33]: [T]he guy who gave her a dose ay the John Knox is the last person she wants tae see .