Old English climban "raise oneself using hands and feet; rise gradually, ascend; make an ascent of" (past tense clamb, past participle clumben, clumbe), from West Germanic *klimban "go up by clinging" (source also of Dutch klimmen, Old High German klimban, German klimmen "to climb").
A strong verb in Old English, weak by 16c. Other Germanic languages long ago dropped the -b. Meaning "to mount as if by climbing" is from mid-14c. Figurative sense of "rise slowly by effort or as if by climbing" is from mid-13c. Related: Climbed; climbing.
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.
"to climb with difficulty using hands and feet," late 14c., possibly frequentative of Middle English climben "to climb" (past tense clamb), or akin to Old Norse klembra "to hook (oneself) on." Related: Clambered; clambering.
"celerity of motion or action," 1650s, from French rapidité and directly from Latin rapiditatem (nominative rapiditas) "swiftness, rapidity, velocity," from rapidus "hasty, swift, rapid" (see rapid).