Lewis & Short

sub-tĭtŭbo, āre, v. n., to stagger, totter, or waver a little (late Lat.).

  1. I. Lit.: pede subtitubo, Ven. Vit. S. Mart. 2, 475.
  2. II. Trop.: subtitubante fide, Prud. Apoth. 651.