Lewis & Short

sessĭbŭlum, i, n. [sedeo], a seat, chair (ante- and post-class.), Plaut. Poen. 1, 2, 56; App. M. 1, p. 112, 30.
The same called sessĭbĭle, is, n., Jul. Val. Rer. Gest. Alex. M. 1, p. 36 Mai.