Rain

The English word for rain has just about always been rain.
 
The word rain (the descent of water in drops through the atmosphere) comes from Middle English rein, Old English regn, Old Saxon regan, Old Norse regn, and Proto-Germanic regna.
 
Regna. Regn. Regan. Rein. Rain.
 
Some old rain-related words and expressions:
Old English renscur (rain shower)
Old English rendropa (raindrop)
Old English reboga (rainbow)
 
“To know enough to come in out of the rain” is from the 1590s.
“To rain cats and dogs” is from 1738.
 
Rain-gauge, an instrument for collecting and measuring rainfall at a given place, is from 1769.
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/
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