Two letters could hardly be closer than N and M, fraternal twins in shape, name, sound and positioning...
Most of N's life has been lived in relation to M ... the 13th letter of the Phoenician alphabet of 1000 B.C. was the wavy-lined mem, ancestor of our M. The name mem meant "water". The Phoenicians' 14th letter was their N, called nun, meaning "fish". Yet the shape of nun was a serpentine undulation, not at all suggesting a fish, aside from an eel.
Modern scholarship has determined that N was invented (by Semitic soldiers or laborers in Egypt) through copying of an Egyptian hieroglyphic picture of a snake. The shape of the Semitic N looked like a snake. The attested letter name "fish" therefore becomes problematic.
In ancient Semitic language, one word for snake was nahash, which began with the sound "n". By the rules of ancient Semitic letter names, this would have been a perfect name for a snaky-looking letter that took the "n" sound. So why didn't the early alphabet users just call their wiggly N letter by the name "snake"?
The answer may be: They did. They did perhaps call the letter nahash at the start, around 2000 or 1900 B.C., but later the name got changed - for the sake of M. Perhaps, in the centuries after 1900 B.C., a need was felt to bring the letters M ("water") and N ("snake") more into line with each other, on account of their unique nasal factor. The two letters had been placed together in sequence at the alphabet's invention; now they would develop even closer associations. In these centuries, the N letter grew more to look like the M letter. And if N's name had been "snake" (as is theorized), then it now changed to "fish", to better fit mem's "water". The purpose of such associations would have been to supply a memory aid for Semitic children learning the alphabet: In the letters' sequence, "fish" would follow "water", the two letters looking and sounding rather similar.
Steinberg points out that in the ancient Ethiopian alphabet Ge'ez the word for the N sound is nahas נחס , which is related to the Hebrew nahash נחש .
Klein writes that the word nun נון meaning "fish" derives from the root נין meaning "to sprout, increase". From the same root we get the word nin נין - which in Modern Hebrew means "great-grandchild". However, in the Tanach, it almost always appears before the word neched נכד - grandchild, as in Bereshit 21:23 וּלְנִינִי וּלְנֶכְדִּי . Therefore, it would appear that this a descendant "before" a grandchild, and in fact, Onkelos translates nin there as "son".
However, there is another root - נון , which means "to waste away, degenerate." Nivun ניוון means "degeneration". Klein quotes Fleischer as claiming that this root derives from the letter name nun and literally means "to be come as lean as the final nun ן ".
As we have seen before, nun alternates with lamed and mem, and it also changes with resh (כנע and כרע ) and (בן and בר).
We have noted many times in the past that the letter nun has a tendency to assimilate or drop out of words (see here here here here and here) - and there should be at least one more this week.
As a suffix, nun can create three kinds of nouns: abstract nouns (בנה - בנין - building), agential nouns (למד -למדן - a learned man), and chemical elements (מים -מימן - hydrogen.)