Subtlety

google

ref

Middle English: from Old French soutilte, from Latin subtilitas, from subtilis ‘fine, delicate’ (see subtle).

wiktionary

ref

From Middle English sotilte, from Old French sutilté, inherited from Latin subtīlitās, from subtīlis(“subtle”). Equivalent to subtle +‎ -ty. Doublet of subtility.

etymonline

ref

subtlety (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.