1670-80; traditionally explained as cul (representing Latinculpābilis guilty) + prit (representing Anglo-Frenchprest ready), marking the prosecution as ready to prove the defendant's guilt. See culpable, presto
1678, from Anglo-Fr. cul prit, contraction of Culpable: prest (d'averrer nostre bille) "guilty, ready (to prove our case)," words used by prosecutor in opening a trial. It seems the abbreviation cul. prit was mistaken in Eng. for an address to the defendant.