subdominant 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).

mid-15c., dominaunt, in ordre dominaunt, the name of the fourth order of angels, from Old French dominant (13c.) and directly from Latin dominantem (nominative dominans), present participle of dominari "to rule, dominate, to govern," from dominus "lord, master," from domus "house" (from PIE root *dem- "house, household").

From 1530s as "exercising rule or chief authority;" by 1854 as "having a controlling effect or influence." Music sense "based on or belonging to the fifth tone of the scale" is from 1819. Sexual bondage sense "exerting control over the submissive partner" is by c. 1960. The noun is first recorded 1819, earliest in the musical sense. Related: Dominantly.