Emily Dickinson Lexicon

SUB'TIL, a. [Fr. subtil; L. subtilis; It. sottile. This word is often written subtle, but less properly.]

1. Thin; not dense or gross; as, subtil air; subtil vapor; subtil medium.
2. Nice; fine; delicate. I do distinguish plain / Each subtil line of her immortal face. – Davies.
3. Acute; piercing; as, subtil pain. – Prior.
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. – Shak.
7. Refined; fine; acute as, a subtil argument.