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).
acute adj.
late 14c., originally of fevers and diseases, "coming quickly to a crisis" (opposed to chronic), from Latin acutus "sharp, pointed," figuratively "shrill, penetrating; intelligent, cunning," past participle of acuere "to sharpen" (literal and figurative), from PIE root *ak- "be sharp, rise (out) to a point, pierce."
It was also used of humors (early 15c.). The meaning "ending in a sharp point" is from 1560s; the sense of "sharp or penetrating in intellect" is from 1580s. Of feelings, pains, etc., "intense," 1727. As a noun, early 15c. of fevers; c. 1600 as "an acute accent." Related: Acutely; acuteness.