SUB'TIL, adjective [Latin 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.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI