subtle

Etymology

From Middle English sotil, soubtil, subtil, borrowed from Old French soutil, subtil, from Latin subtīlis (“fine, thin, slender, delicate”); probably, originally, “woven fine”, and from sub (“under”) + tēla (“a web”), from texere (“to weave”). Displaced native Old English smēag (literally “creeping”).

adj

1. Hard to grasp; not obvious or easily understood.
2. Barely noticeable, not obvious, indistinct.
   The difference is subtle, but you can hear it if you listen carefully.
   The mighty Magnet from the Center darts / This ſtrong, tho' ſubtile Force, thro' all the Parts: / Its active Rays ejaculated thence, / Irradiate all the wide Circumference. 1712, Richard Blackmore, Creation: A Philosophical Poem. Demonstrating the Existence and Providence of a God. In Seven Books, book I, London: Printed for S. Buckley, at the Dolphin in Little-Britain; and J[acob] Tonson, at Shakespear's Head over-against Catherine-Street in the Strand, OCLC 731619916; 5th edition, Dublin: Printed by S. Powell, for G. Risk, G. Ewing, and W. Smith, in Dame's-street, 1727, OCLC 728300884, page 7
3. (of a thing) Cleverly contrived.
4. (of a person or animal) Cunning, skillful.
5. Insidious.
6. Tenuous; rarefied; of low density or thin consistency.
7. (obsolete) Refined; exquisite.