A word of much controversy. We complain about it when it rains, but without it, our crops would struggle and we would be very thirsty!
So where does the word come from? In Middle English we can find reyn and rein, both of which came from Old English regn. The story doesn’t end here, of course. The Old English word developed from Proto-Germanic *regna, the source of words in several other languages: Dutch regen, German Regen, Danish regn, Norwegian regn and Swedish regn.
Can we trace the word any further back? Well, it seems there are two schools of thought. One links “rain” to Proto-Indo-European *(h)reg, meaning “moist, wet”, which is presumed to have given us Latin rigare, to wet, and which would therefore make “irrigate” a cognate.
The other theory is that it comes from Proto-Indo-European *srew, meaning “to flow”. If this is the case, it would make it cognate with Greek ρέω (reo), which means “to flow”, and also with words that at first glance may seem totally unconnected: “stream” and “rhythm”.
Personally, I think there is no reason why we cannot theorise that the two Proto-Indo-European roots are linked, perhaps going back to an earlier form that gave rise to the two groups.