How did Tolkien and Lewis come to use the similar names "neekerbreeker" and "Nikabrik"?

Question

I noticed today, after being reminded of the episode in The Fellowship of the Ring, when Strider and the hobbits are crossing the Midgewater Marshes, where they hear countless noisy insects,

There were thousands of them, and they squeaked all round, neek-breek, breek-neek...

for which Sam coins the name "neekerbreekers," the name is awfully similar to the name of the Black Dwarf in Prince Caspian: Nikabrik (especially given the non-rhotic English spoken by the two Oxford don authors).

The similarity seems unlikely to be a coincidence, since Tolkien and Lewis were good friends. However, I am wondering whether there is any more information about the similar names, especially who came up with the sound first. Prince Caspian was published in 1951, so after most of The Fellowship of the Ring was written, but before it was published. Given this timeframe, and since Tolkien’s usage has an in-story onomatopoetic origin, while Lewis’s apparently does not (and as this blog post points out, the way Tolkien uses the name is also suggestive of nicker for a water spirit), my initial guess would be that Tolkien coined the name first, but I wonder if there is any additional information either way.

Answer 1

C. S. Lewis is already known to have borrowed words from Tolkien.

As you may recall, Tolkien and Lewis were part of an Oxford literary group called the "Inklings". At their meetings, members would sometimes read the stuff they were working on to each other.

As related by Tolkien in Letter 169:

Your discovery of 'Numinor' in C.S.L.'s That Hideous Strength is discovery of a plagiarism: well, not that, since he used the word, taken from my legends of the First and Second Ages, in the belief that they would soon appear. They have not, but I suppose now they may. The spelling Numinor is due to his hearing it and not seeing it.

There are several references to "Numinor" in That Hideous Strength:

“What common measure is there,” he would ask, “between ceremonial occultists like Faustus and Prospero and Archimago with their midnight studies, their forbidden books, their attendant fiends or elementals, and a figure like Merlin who seems to produce his results simply by being Merlin?” And Ransom agreed. He thought that Merlin’s art was the last survival of something older and different — something brought to Western Europe after the fall of Numinor and going back to an era in which the general relations of mind and matter on this planet had been other than those we know.

But Lewis acknowledges this in his preface:

Those who would like to learn further about Numinor and the True West must (alas!) await the publication of much that still exists only in the MSS. of my friend, Professor J.R.R. Tolkien.

Tolkien's Alkallabêth, or the "Downfall of Númenor," started as part of an aborted joint project where Tolkien would write a "time travel" story and Lewis would tell a "space travel" story. The "space travel" story resulted in Lewis's Space Trilogy: Out of the Silent Planet, Perelandra, and That Hideous Strength.

Alkallabêth, as you probably know, was only published posthumously, as part of his son Christopher's best reconstruction of Tolkien's legendarium, but LotR contains references to Númenor.