The story
From Anglo-Norman cul. prit, contraction of culpable: prest (d'averrer nostre bille) 'guilty: ready (to prove our case)', words used by prosecutor in opening a trial, mistaken in English for an address to the defendant. See culpable. It may have been influenced by Latin culpa (“blame, fault”).
Etymology from Wiktionary, CC BY-SA 4.0.