a corruption of Cupar, etc., is trying a man after execution. Similar expressions are Jedwood, Jeddart, and Jedburgh justice. Cowper justice had its rise from a baron-baile in Coupar-Angus, before heritable jurisdictions were abolished. (See Lydford Law.)
“Cowper Law, as we say in Scotland—hang a man first, and then judge him.” —Lord de Ros: Tower of London.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894