subtext n.

word-forming element meaning "under, beneath; behind; from under; resulting from further division," from Latin preposition sub "under, below, beneath, at the foot of," also "close to, up to, towards;" of time, "within, during;" figuratively "subject to, in the power of;" also "a little, somewhat" (as in sub-horridus "somewhat rough"), from PIE *(s)up- (perhaps representing *ex-upo-), a variant form of the root *upo "under," also "up from under." The Latin word also was used as a prefix and in various combinations.

In Latin assimilated to following -c-, -f-, -g-, -p-, and often -r- and -m-. In Old French the prefix appears in the full Latin form only "in learned adoptions of old Latin compounds" [OED], and in popular use it was represented by sous-, sou-; as in French souvenir from Latin subvenire, souscrire (Old French souzescrire) from subscribere, etc.

The original meaning is now obscured in many words from Latin (suggest, suspect, subject, etc.). The prefix is active in Modern English, sometimes meaning "subordinate" (as in subcontractor); "inferior" (17c., as in subhuman); "smaller" (18c.); "a part or division of" (c. 1800, as in subcontinent).

late 14c., "wording of anything written," from Old French texte, Old North French tixte "text, book; Gospels" (12c.), from Medieval Latin textus "the Scriptures, text, treatise," in Late Latin "written account, content, characters used in a document," from Latin textus "style or texture of a work," literally "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"). 

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." The meaning "a digital text message" is by 2005.