The Collaborative International Dictionary of English v.0.48:
Acidulous \A*cid"u*lous\, a. [L. acidulus, dim. of acidus. See Acid.] Slightly sour; sub-acid; sourish; as, an acidulous tincture. --E. Burke. [1913 Webster] Acidulous mineral waters, such as contain carbonic anhydride. [1913 Webster]