Anglo-French cul (abbr of culpable culpable) + prest, prit ready (from Latin praestus), prob from the legal phrase Cupable; prest d'averer (you are) guilty; (we are) ready to prove it. In English courts after the Norman Conquest, a prisoner's plea of not guilty was answered by the clerk of the crown, who said culpable (guilty) and that he was prest (ready) to proceed with the prosecution. The formula cul. prest or cul. prit, was then entered on the official record of proceedings, was later. Later, these words were mistakenly taken as a single word referring to the prisoner (the earliest record of this use is not before the late 17th cent.)