Subtle

Sly in design; artful; cunning; insinuating; subtile; -- applied to persons; as, a subtle foe.

Cunningly devised; crafty; treacherous; as, a subtle stratagem.

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.

Smooth and deceptive.

difficult to detect or grasp by the mind or analyze; "his whole attitude had undergone a subtle change"; "a subtle difference"; "that elusive thing the soul"