E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 Scotlandhang a man first, and then judge him.Lord de Ros: Tower of London.