TgO Num15:34 :וַאֲסַרֻו יָתֵיה בְבֵית מַטְרָא אֲרֵי לָא אִתפְרֵיש לְהוֹן מָא דְיַעבְדֻון לֵיה‏ : they kept him in custody for it had never been explained to them what to do to him
See the entire entry for maṭṭar, maṭṭrā n.m. prison, watch