subtitle 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).

c. 1300, "inscription, heading," from Old French title "title or chapter of a book; position; legal permit" (12c., Modern French titre, by dissimilation), and in part from Old English titul, both from Latin titulus "inscription, label, ticket, placard, heading; honorable appellation, title of honor," of unknown origin. Meaning "name of a book, play, etc." first recorded mid-14c. The sense of "name showing a person's rank" in English is first attested 1580s. Sports championship sense attested from 1913 (originally in lawn tennis), hence titlist (1913). A title role in theater is one which gives its name to the play.