An Etymological Dictionary of Astronomy and Astrophysics

1. A brief, abrupt precipitation from a cloud, characterized by the suddenness with which it begins and ends and by the rapid changes in intensity.
   

2. An abundant flow; an outpouring, such as → cosmic-ray shower; → air shower; → meteor shower.

Etymology (EN): M.E. shour; O.E. scur “short fall of rain, fall of missiles or blows;” cf. O.N. skur, O.S., O.H.G. scur, Ger. Schauer.

Etymology (PE): Ragbâr “cloudburst,” from rag “cloudburst; thunder,” → rain + bâr “raining, rain,” from bâridan,
→ rain.
Tondbâr, with tond “fast,” → velocity.