Light as a feather, or better, thin as gossamer.
Either of these expressions describes fairly accurately the gentleness which is characteristic of true subtlety, but “gossamer” is a close translation of the Latin word from which subtle originates.
Subtle was brought into English from the Old French soutil (alternately spelled soul or sutil) with the meaning of “thin (rarefied), fine, delicate.”
But the French word came from the Latin subtilis, a contraction of subtexilis, “finely woven” (literally, “underwoven”), from sub, “under,” and texare, “to weave.”
For many years an alternate spelling of the English word, subtile, existed in parallel with the presently accepted spelling, this being based directly on the Latin ancestor.