SUB'TIL, a. [L. subtilis. This word is often written subtle, but less properly.]
1. Thin; not dense or gross; as subtil air; subtil vapor; a subtil medium.2. Nice; fine; delicate. I do distinguish plain Each subtil line of her immortal face.3. Acute; piercing; as subtil pain.4. Sly; artful; cunning; crafty; insinuating; as a subtil person; a subtil adversary.5. Planned by art; deceitful; as a subtil scheme.6. Deceitful; treacherous.7. Refined; fine; acute; as a subtil argument.