What is the X in X-mas?
1765, from French rapides (see rapid); applied by French voyagers to rough, swift-flowing reaches in North American rivers.
1630s, "moving quickly," from French rapide (17c.) and directly from Latin rapidus "hasty, swift, rapid; snatching; fierce, impetuous," from rapere "hurry away, carry off, seize, plunder," from PIE root *rep- "to snatch" (cf. Greek ereptomai "devour," harpazein "snatch away," Lithuanian raples "tongs"). Meaning "happening in a short time" is from 1780. Related: Rapidly; rapidness. Rapid-transit first attested 1852, in reference to street railways; rapid eye movement is from 1906.