The Collaborative International Dictionary of English v.0.48:

Ingenious \In*gen"ious\, a. [L. ingeniosus, fr. ingenium innate
   or natural quality, natural capacity, genius: cf. F.
   ing['e]nieux. See Engine.]
   1. Possessed of genius, or the faculty of invention; skillful
      or promp to invent; having an aptitude to contrive, or to
      form new combinations; as, an ingenious author, mechanic.
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            A man . . . very wise and ingenious in feats of war.
                                                  --Hakluyt.
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            Thou, king, send out
            For torturers ingenious.              --Shak.
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            The more ingenious men are, the more apt are they to
            trouble themselves.                   --Sir W.
                                                  Temple.
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   2. Proceeding from, pertaining to, or characterized by,
      genius or ingenuity; of curious design, structure, or
      mechanism; as, an ingenious model, or machine; an
      ingenious scheme, contrivance, etc.
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            Thus men go wrong with an ingenious skill. --Cowper.
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   3. Witty; shrewd; adroit; keen; sagacious; as, an ingenious
      reply.
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   4. Mental; intellectual. [Obs.]
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            A course of learning and ingenious studies. --Shak.
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WordNet (r) 3.0 (2006):

ingenious
    adj 1: showing inventiveness and skill; "a clever gadget"; "the
           cunning maneuvers leading to his success"; "an ingenious
           solution to the problem" [syn: clever, cunning,
           ingenious]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "ingenious":
   Daedalian, Machiavellian, Machiavellic, acute, adept, adroit, apt,
   arch, artful, artistic, astute, authoritative, bravura, bright,
   brilliant, cagey, canny, clean, clever, conceptive, conceptual,
   coordinated, crack, crackerjack, crafty, creative, cunning, cute,
   daedal, deceitful, deep, deep-laid, deft, demiurgic, designing,
   dexterous, dextrous, diplomatic, esemplastic, excellent, expert,
   fancy, fecund, feline, fertile, foxy, germinal, gifted, good,
   goodish, graceful, guileful, handy, ideational, ideative,
   imaginative, innovational, innovative, insidious, inspired,
   inventive, keen, knowing, magisterial, masterful, masterly, neat,
   no mean, notional, on the ball, original, originative, pawky,
   politic, pregnant, productive, professional, proficient, prolific,
   quick, quite some, ready, resourceful, scheming, seminal,
   serpentine, shaping, sharp, shifty, shrewd, skilled, skillful,
   slick, slim, slippery, sly, smart, smooth, snaky, sneaky, some,
   sophistical, statesmanlike, stealthy, strategic, stylish, subtile,
   subtle, supple, tactful, tactical, talented, teeming, the compleat,
   the complete, trickish, tricksy, tricky, virtuoso, visioned,
   vulpine, wary, well-done, wily, workmanlike