subtility (n.)

c. 1300, sotilte, "skill, ingenuity," from Old French sotilte "skillfulness, cunning" (Modern French subtilité), from Latin subtilitatem (nominative subtilitas) "fineness; simplicity, slenderness," noun of quality from subtilis "fine, thin, delicate" (see subtle). From late 14c. as "cleverness, shrewdness; trickery, guile, craftiness," also "thinness, slenderness, smallness; rarity." The -b- begins to appear late 14c. in English, in imitation of Latin.

c. 1300, sotil; modern form from late 14c., "clever, dexterous, crafty; not dense, thin, rarefied," from Old French subtil (14c.), a learned Latinized reformation of earlier sotil (12c.), source of subtle (q.v.). Still used in some Bible translations in Genesis iii.1, and it survived after 17c. as a parallel formation to subtle in some material senses ("fine, delicate, thin"). Related: Subtilly.