'Prison' meant 'the arrest,' not the building — from Latin 'prehendere' (to grasp). Kin to 'comprehend.'
A building in which people are legally held as punishment for a crime or while awaiting trial.
From Old French 'prisun' (prison, captivity), from Latin 'prēnsiō' or 'prēhēnsiō' (a seizing, an arrest), from 'prehendere' (to seize, to grasp, to take hold of), from PIE *ghend- (to seize, to take). A prison was originally the act of seizing -- the arrest itself -- before it became the place of confinement. The same root produced 'apprehend' (to seize), 'comprehend' (to grasp mentally
'Prison,' 'comprehend,' 'surprise,' and 'enterprise' all come from Latin 'prehendere' (to seize). A prison is where you are seized. To comprehend is to seize with the mind. A surprise seizes you unexpectedly (from Old French 'surprendre'). An enterprise is something seized upon, undertaken. Even 'prehensile' (as in a monkey's tail) means