pow

"person confined in a prison, captive person," mid-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.)) and from Medieval Latin prisonarius.

Figurative sense of "one who is deprived of liberty or kept in restraint" is from late 14c. Captives taken in war have been called prisoners since late 14c., but the phrase prisoner of war dates from 1670s (see also POW). The children's game prisoner's base is attested as such by 1590s (prison base); the logic problem of the prisoner's dilemma is attested by that name from 1957.