Subtle

1. superl. Smooth and deceptive.
2. superl. Sly in design; artful; cunning; insinuating; subtile; — applied to persons; as, a subtle foe.
3. superl. Cunningly devised; crafty; treacherous; as, a subtle stratagem.
4. superl. 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.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.