means the quality of being delicate, understated, or finely nuanced — achieving an effect without obvious force or display.

from From Latin subtilis, literally "finely woven" — from sub- ("under") and tela ("web" or "warp of threads on a loom"). The image is of a fabric spun so fine you can barely see the threads, which is exactly the point. It reached English through Old French sotil/subtil in the medieval period, picking up the spelling that fakes a closer Latin look, and kept the sense of something so finely made it slips past notice.