The Collaborative International Dictionary of English v.0.48:

Subtilty \Sub"til*ty\, n. [Contr. fr. subtility.]
   1. The quality or state of being subtile; thinness; fineness;
      as, the subtility of air or light.
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   2. Refinement; extreme acuteness; subtlety.
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            Intelligible discourses are spoiled by too much
            subtility in nice divisions.          --Locke.
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   3. Cunning; skill; craft. [Obs.]
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            To learn a lewd man this subtility.   --Chaucer.
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   4. Slyness in design; artifice; guile; a cunning design or
      artifice; a trick; subtlety.
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            O full of all subtility and all mischief. --Acts
                                                  xiii. 10.
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   Note: In senses 2, 3, and 4 the word is more commonly written
         subtlety.
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