sub·tle / ˈsətl/ • adj. (-tler, -tlest) (esp. of a change or distinction) so delicate or precise as to be difficult to analyze or describe: his language expresses rich and subtle meanings. ∎  (of a mixture or effect) delicately complex and understated: subtle lighting. ∎  making use of clever and indirect methods to achieve something: he tried a more subtle approach. ∎  capable of making fine distinctions: a subtle mind. ∎  arranged in an ingenious and elaborate way. ∎ archaic crafty; cunning.DERIVATIVES: sub·tle·ness n.sub·tly adv.

subtle of thin or fine consistency; marked by acumen or fine discrimination. XIV. ME. sutil, sotil — OF. sutil, so(u)til — L. subtīlis, prob. for *subtexlis, *subtēlis, f. *sub texlā, *sub tēlā (passing) under the warp (weaver's term).
So subtlety (-TY2) XIV. — OF. s(o)utilté :- L. subtilitātem. Hence subtly XIV. The var. subtil(e) (- OF. subtil, latinized form of sutil) arose in late ME. and remained till modern times along with subtil(i)ty in arch. or affected use.