Source : Webster's Revised Unabridged Dictionary (1913)

Subacid \Sub*ac"id\, a. [L. subacidus. See {Sub-}, {Acid}.]
   Moderately acid or sour; as, some plants have subacid juices.
   -- n. A substance moderately acid.

Source : WordNet®

subacid
     adj : slightly sour to the taste