The Collaborative International Dictionary of English v.0.48:

Acumen \A*cu"men\, n. [L. acumen, fr. acuere to sharpen. Cf.
   Acute.]
   Quickness of perception or discernment; penetration of mind;
   the faculty of nice discrimination. --Selden.
   [1913 Webster]

   Syn: Sharpness; sagacity; keenness; shrewdness; acuteness.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "acumen":
   acuity, acuteness, apperception, astuteness, cogency,
   critical discernment, discernment, discrimination, farseeingness,
   farsightedness, flair, foresight, foresightedness, incisiveness,
   insight, judgment, keenness, longheadedness, longsightedness,
   penetration, perception, perceptiveness, percipience,
   perspicaciousness, perspicacity, perspicuity, perspicuousness,
   providence, sagaciousness, sagacity, sensibility, sharpness,
   shrewdness, trenchancy