rapidity (n.)

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.