subtle (adj.)

c. 1300, sotil; the modern form is attested from late 14c., "clever, dexterous, crafty;" of fluids, "not dense, thin, rarefied;" of fabrics, "of fine or delicate texture;" from Old French subtil (14c.), a learned Latinized reformation of earlier sotil (12c.), source of subtle (q.v.).

It is a doublet or variant of subtle, originally used in all the same senses. Some lines of Chaucer that have the word alternate between the two spellings in different transcriptions. And compare subtilty, a late 14c. variant of subtlety altered by influence of this word.

Subtile still is used in some Bible translations in Genesis iii.1, and it survived past 17c. in some material senses ("fine, delicately constituted, thin") as a parallel formation to subtle. Related: Subtilly; subtilely.

late 14c., "the wording of anything written," from Old French texte, Old North French tixte "text, book; Gospels" (12c.), from Medieval Latin textus "the Scriptures; a text, a treatise," earlier, in Late Latin "written account, content, characters used in a document," from Latin textus "style or texture of a work," etymologically "thing woven," from past-participle stem of texere "to weave, to join, fit together, braid, interweave, construct, fabricate, build" (from PIE root *teks- "to weave, to fabricate, to make; make wicker or wattle framework"). 

Also in English from late 14c. more specifically as "an authoritative writing or document; a translated discourse or composition (as opposed to the commentary on it); story, tale, narrative; Christian doctrine; a passage of the Bible (as a proof or a subject of discourse); the letter of the Scriptures," especially in the original language.

Hence, generally, "a subject, theme" (c. 1600), figuratively, from the notion of "where one starts." The meaning "a digital text message" is by 2005.

An ancient metaphor: thought is a thread, and the raconteur is a spinner of yarns — but the true storyteller, the poet, is a weaver. The scribes made this old and audible abstraction into a new and visible fact. After long practice, their work took on such an even, flexible texture that they called the written page a textus, which means cloth. [Robert Bringhurst, "The Elements of Typographic Style"]

To Socrates, a word (the name of a thing) is "an instrument of teaching and of separating reality, as a shuttle is an instrument of separating the web" [Cratylus].