"person in prison, captive person," late 14c. (earlier "a jailer," mid-13c., but this did not survive Middle English), from Old French prisonier "captive, hostage" (12c., Modern French prisonnier), from prisoun (see prison, n.). Captives taken in war have been called prisoners since mid-14c.; phrase prisoner of war dates from 1670s (see also POW). Prisoner's dilemma attested from 1957.