The Collaborative International Dictionary of English v.0.48:

Acetous \A*ce"tous\ (#; 277), a. [L. acetum vinegar, fr. acere
   to be sour.]
   1. Having a sour taste; sour; acid. "An acetous spirit."
      --Boyle. "A liquid of an acetous kind." --Bp. Lowth.
      [1913 Webster]

   2. Causing, or connected with, acetification; as, acetous
      fermentation.
      [1913 Webster]

   Acetous acid, a name formerly given to vinegar, which is a
      dilute solution of acetic acid.
      [1913 Webster +PJC]