circular (n.)
1550s, "circular figure," from circular (adj.). Meaning "a notice circulated" is from 1818.
circular (adj.)
late 14c., from Anglo-French circuler, Old French circuler "circular" (14c., Modern French circulaire), from Latin circularis, from circulus (see circle (n.)). The metaphoric circular firing squad is attested by 1990.