subtlety n.

c. 1300 (mid-13c. as a surname), sotil, "penetrating; ingenious; refined" (of the mind); "sophisticated, intricate, abstruse" (of arguments), from Old French sotil, soutil, subtil "adept, adroit; cunning, wise; detailed; well-crafted" (12c., Modern French subtil), from Latin subtilis "fine, thin, delicate, finely woven;" figuratively "precise, exact, accurate," in taste or judgment, "fine, keen," of style, "plain, simple, direct," from sub "under" (see sub-) + -tilis, from tela "web, net, warp of a fabric," from PIE root *teks- "to weave," also "to fabricate." According to Watkins, the notion is of the "thread passing under the warp" as the finest thread.

From early 14c. in reference to things, "of thin consistency;" in reference to craftsmen, "cunning, skilled, clever;" Depreciative sense "insidious, treacherously cunning; deceitful" is from mid-14c. Material senses of "not dense or viscous, light; pure; delicate, thin, slender; fine, consisting of small particles" are from late 14c. sotil wares were goods sold in powdered form or finely ground. Partially re-Latinized in spelling, and also by confusion with subtile.

late 14c., "acuteness, skill, cunning," alteration of subtlety (q.v.) on model of subtile, or else from Old French subtilite, from Latin subtilitas "fineness, simplicity."