surreptitious adj.

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).

1630s, "moving or doing quickly, capable of great speed," from French rapide (17c.) and directly from Latin rapidus "hasty, swift; snatching; fierce, impetuous," from rapere "hurry away, carry off, seize, plunder," from PIE root *rep- "to snatch" (source also of Greek ereptomai "devour," harpazein "snatch away," Lithuanian raplės "tongs").

Meaning "happening in a short time, coming quickly into existence" is from 1780. Related: Rapidly; rapidness. Rapid-fire (adj.) 1890 in reference to guns, figurative or transferred use by 1900; the noun phrase is by 1836. Rapid-transit first attested 1852, in reference to street railways; rapid eye movement, associated with a certain phase of sleep, is from 1906.

"act of obtaining a favor by fraudulent suppression of facts," c. 1600, from Latin subreptionem (nominative subreptio), noun of action from past-participle stem of subripere, surripere (see surreptitious). Related: Subreptitious.