The Collaborative International Dictionary of English v.0.48:

Sagacious \Sa*ga"cious\, a. [L. sagax, sagacis, akin to sagire
   to perceive quickly or keenly, and probably to E. seek. See
   Seek, and cf. Presage.]
   1. Of quick sense perceptions; keen-scented; skilled in
      following a trail.
      [1913 Webster]

            Sagacious of his quarry from so far.  --Milton.
      [1913 Webster]

   2. Hence, of quick intellectual perceptions; of keen
      penetration and judgment; discerning and judicious;
      knowing; far-sighted; shrewd; sage; wise; as, a sagacious
      man; a sagacious remark.
      [1913 Webster]

            Instinct . . . makes them, many times, sagacious
            above our apprehension.               --Dr. H. More.
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            Only sagacious heads light on these observations,
            and reduce them into general propositions. --Locke.
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   Syn: See Shrewd.
        [1913 Webster] -- Sa*ga"cious*ly, adv. --
        Sa*ga"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sagaciously
    adv 1: in a shrewd manner; "he invested his fortune astutely";
           "he was acutely insightful" [syn: astutely, shrewdly,
           sagaciously, sapiently, acutely]