Lewis & Short

mĕdĭastīnus, i, m. [medius], a common servant, drudge, employed in all kinds of menial occupations (class.): tu illi mediastinus, Cato ap. Non. 143, 9: atque bubulcus, Lucil. ib. 7: exercitus collectus ex senibus desperatis, ex agresti luxuria, ex rusticis mediastinis, decoctoribus, Cic. Cat. 2, 3, 5 (dub.): tu mediastinus tacitā prece rura petebas, Hor. Ep. 1, 14, 14; Col. 2, 13, 7; 1, 9, 3: Prodicus instituens quam vocant iatralepticen, reunctoribus quoque medicorum, ac mediastinis vectigal invenit, i. e. medical assistants, Plin. 29, 1, 2, § 4; Dig. 4, 9, 1, § 5.