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).
1550s, "continuous tract of land," from continent land (mid-15c.), translating Medieval Latin terra continens "continuous land," from Latin continens "continuous," present participle of continere "to hold together, enclose," from assimilated form of com "with, together" (see con-) + tenere "to hold" (from PIE root *ten- "to stretch").
As "one of the large land masses of the globe" from 1610s. As "the mainland of Europe" (from the point of view of Britain), from c. 1600.
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Definitions of subcontinent from WordNet
a large and distinctive landmass (as India or Greenland) that is a distinct part of some continent;