Sub·tle a. [Compar. Subtler superl. Subtlest ]
 1. Sly in design; artful; cunning; insinuating; subtile; -- applied to persons; as, a subtle foe. “A subtle traitor.”
 2. Cunningly devised; crafty; treacherous; as, a subtle stratagem.
 3. Characterized by refinement and niceness in drawing distinctions; nicely discriminating; -- said of persons; as, a subtle logician; refined; tenuous; sinuous; insinuating; hence, penetrative or pervasive; -- said of the mind; its faculties, or its operations; as, a subtle intellect; a subtle imagination; a subtle process of thought; also, difficult of apprehension; elusive.
    Things remote from use, obscure and subtle.   --Milton.
 4. Smooth and deceptive. [Obs.]
    Like to a bowl upon a subtle ground [bowling ground].   --Shak.
 Syn: -- Artful; crafty; cunning; shrewd; sly; wily.
 Usage: Subtle is the most comprehensive of these epithets and implies the finest intellectual quality. See Shrewd, and Cunning.

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