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.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
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Harper, D. (n.d.). Etymology of rapacious. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/rapacious