Lewis & Short

dŭbĭtābĭlis, e, adj. [dubito], doubtful (very rare).

  1. I. Pass., to be doubted: verum, Ov. M. 1, 223: virtus, id. ib. 13, 21.
  2. II. Act., feeling doubt: pectus, Prud. Apoth. 649.