subsection 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., seccioun, in astronomy, "the intersection of two straight lines; a division of a scale;" from Old French section and directly from Latin sectionem (nominative sectio) "a cutting, cutting off, division," noun of action from past-participle stem of secare "to cut" (from PIE root *sek- "to cut").

 The meaning "a part cut off or separated from the rest" is from early 15c. That of "a drawing representing something as if cut through" is from 1660s. From 1550s in English in the meaning "act of cutting or dividing," a sense now rare or archaic and preserved in some medical phrases, most notably Caesarian section. The meaning "a subdivision of a written work, statute, etc." is from 1570s.

Books are commonly divided into Chapters, Chapters into Sections, and Sections into Paragraphs or Breaks, as Printers call them .... [Blount, "Glossigraphia," 1656]

In music, "a group of similar instruments in a band or orchestra" (1880). In U.S. history, a square of 640 acres into which public lands were divided (1785). In World War II U.S. military slang, section eight was a reference to the passage in an Army Regulations act that referred to discharge on grounds of insanity.