Noah Webster's New International Dictionary of the English Language

1. (a.) Thin; not dense or gross; rare; as, subtle air; subtle vapor; a subtle medium.

2. (a.) Delicately constituted or constructed; nice; fine; delicate; tenuous; finely woven.

3. (a.) Acute; piercing; searching.

4. (a.) Characterized by nicety of discrimination; discerning; delicate; refined; subtle.

5. (a.) Sly; artful; cunning; crafty; subtle; as, a subtle person; a subtle adversary; a subtle scheme.