Origins
The word 'prison' preserves a moment of violent physicality at the heart of the justice system.โโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโ It descends not from a word meaning 'building' or 'cage' but from a word meaning 'the act of seizing' -- the grab, the arrest, the moment a person is taken. The building came later; the seizure came first.
English borrowed the word around 1121 from Old French 'prisun' (prison, captivity, the state of being held), which descends from Latin 'prฤnsiล' (a taking, a seizing, an arrest), a contraction of 'prฤhฤnsiล,' the noun form of 'prehendere' (to seize, to grasp, to lay hold of). The Latin verb itself derives from PIE *ghend- (to seize, to take), possibly with the prefix *prae- (before, in front of) -- to seize something in front of you, to get hold of it.
The semantic shift from 'seizing' to 'place of confinement' happened during the Old French period. The word moved from naming the action (the arrest) to naming the condition (captivity) to naming the place where the condition was suffered (the jail). This trajectory -- from event to state to location -- is common in the history of institutional vocabulary.
Latin Roots
The Latin verb 'prehendere' generated an enormous family in English, mostly through French. 'Apprehend' (ad- + prehendere, to seize upon) means both 'to arrest' and 'to understand' -- the physical and mental senses of grasping. 'Comprehend' (com- + prehendere, to seize together, to grasp entirely) took the mental path exclusively. 'Reprehend' (re- + prehendere, to seize back, to hold back) means 'to rebuke,' and 'reprehensible' describes conduct deserving of censure. 'Prehensile' (capable of grasping) is used of monkey tails and chameleon tongues -- anatomy that seizes.
More surprising derivatives arrived through French phonological changes that obscured the Latin root. 'Surprise' comes from Old French 'surprendre' (sur- + prendre, 'to seize from above, to overtake'), from Latin 'superprehendere.' A surprise is an over-seizure, something that grabs you from above. 'Enterprise' comes from Old French 'entreprendre' (entre- + prendre, 'to seize between, to undertake'), from Latin 'inter' + 'prehendere.' An enterprise is something grasped between the hands, undertaken.
The Old French verb 'prendre' (to take) -- the descendant of Latin 'prehendere' after severe phonological erosion -- is one of the most common verbs in French and the source of many English borrowings that no longer look related to 'prison' at all. 'Prize' (something seized), 'prey' (something seized by a predator, though this also has influence from Latin 'praeda'), and 'predatory' all touch the same semantic field of seizing.
French Influence
The coexistence of 'prison' and 'jail' in English reflects the language's dual Norman and native inheritance. 'Jail' (from Old French 'jaiole,' from Late Latin 'caveola,' diminutive of 'cavea,' cage) entered English from a different French dialect. American English tends to use 'jail' for local or short-term facilities and 'prison' for state or federal institutions serving longer sentences, but the distinction is conventional rather than etymological.