“There were also abominable creatures haunting the reeds and tussocks that from the sound of them were evil relatives of the cricket. There were thousands of them, and they squeaked all round, neek-breek, breek-neek, unceasingly all the night, until the hobbits were nearly frantic.” Neekerbreekers (as Sam calls them) are an incessantly noisy insect species inhabiting the Midgewater Marshes, about three days’ east of Bree. In the “Nomenclature of The Lord of the Rings”, Tolkien explains that this is “[a]n invented insect-name” and that translators should render it by an “invention of similar sound (supposed to be like that of a cricket)” .
This is straightforward enough. Tolkien suggests the name is onomatopoeic. As Steve Walker succinctly puts it: “Neekerbreekers sound their name” . My friend Mark Hooker has aptly noted a parallel in H. Rider Haggard. In his novel She, there are “sullen peaty pools” filled with “musqueteers”, “tens of thousands of the most blood-thirsty, pertinacious, and huge mosquitoes” . What else can I add to these clear-cut comments? Maybe a bit more.
First, consider that the word mosquito itself, a Spanish diminutive of Latin musca “fly, gnat”, is thought to be imitative in origin too (cp. Greek μύζειν “to mutter”). Cognates in the Germanic languages include Old High German mucca, Middle High German mücke, Middle Dutch mugge, Old Saxon muggia, Old Norse mý, and Old English mycg, from which we derive the Modern English midge — as in Midgewater Marshes. That’s rather a nice coincidence, and possibly a bit of ammunition for Mark’s case that Tolkien may have had Haggard in mind.
Second, something else struck me recently. This is a bit more of a stretch, but I offer it as food for thought. Consider this passage from Laȝamon’s Brut —
Þat is a seolcuð mere | iset a middel-ærdeFor those whose Early Middle English is a bit rusty: “It is a strange lake, set in Middle-earth, with marsh and with reed, with waters exceedingly broad, with fish and with fowl, with evil things. The water is immensely wide, nickers bathe in it, there elves play in the dreadful pool.”
mid fenne & mid ræode | mid watere swiðe bræde
mid fiscen & mid feoȝelen | mid uniuele þingen
Þat water is unimete | brade nikeres þer ba[ð]ieð inne
þer is æluene ploȝe | in atteliche pole.
The passage has the Dead Marshes dead to rights, don’t you think? But perhaps there is a hint of the Midgewater Marshes with its neekerbreekers as well. After all, what are these Middle English nikeres, which I translated above as nickers?
The word usually means something like a water-monster, sprite, sea-goblin, siren, mermaid, etc., depending on the tale in which it appears. It is the source of the folkloric nixie (a kind of water sprite), and it has cognates in all the Germanic tongues — e.g., MD nicker, ON nykr, OHG nichus, and OE nicor. The latter has been glossed as hippopotamus and crocodile, but OE nicor, as well as the compound nicor-hús “nicker-house”, occur throughout Beowulf to describe sea monsters and their lairs. Indeed, the haunted mere in Laȝamon’s Brut is sometimes compared in the scholarly literature to the abode of Grendel’s dam in Beowulf. As C.S. Lewis put it: “[Laȝamon’s] nikeres and their pool might have come straight out of Beowulf.” 
The word survived into Modern English, spelled nicker, though it has been obsolete for a long time now. The Oxford English Dictionary defines it as “a supernatural being supposed to live in the sea or other waters; a water-demon, a kelpie. Formerly also (in Middle English): a siren, a mermaid (obs.).” E.R. Eddison used it as late as 1922 in The Worm Ouroboros: “on the walls strange portraitures: lions, dragons, nickers of the sea, spread-eagles, elephants, swans, unicorns” , but otherwise, the word is all but dead.
Is there any reason to think Tolkien had this word in the back of his mind when he invented the neekerbreekers? Not a strong reason, certainly, though it’s fun to imagine he might have. Why not? The neekerbreeker is an abominable creature inhabiting a marshy region in Middle-earth — and precisely the same could be said of the nicker, nikere, nicor, however you wish to spell it. Admittedly, “evil relatives of the cricket” are not quite the same as water-demons, but the phonological envelopes of both the real-world word and the first part of Tolkien’s are identical. The second half is probably an imitative reduplication, not at all uncommon in English.
In any case, I think it’s fair to say neekerbreekers are best avoided. They might be no more than noisy crickets, but maybe not. Better safe than sorry. ;)
 J.R.R. Tolkien. The Lord of the Rings. 50th anniversary ed. Boston: Houghton Mifflin, 2004, p. 183.
 J.R.R. Tolkien. “Nomenclature of The Lord of the Rings.” The Lord of the Rings: A Reader’s Companion. Ed. Wayne G. Hammond and Christina Scull. Boston: Houghton Mifflin, 2005, p. 760.
 Steve Walker. The Power of Tolkien’s Prose: Middle-earth’s Magical Style. New York: Palgrave Macmillan, 2009, p. 23.
 Mark T. Hooker. A Tolkienian Mathomium: A Collection of Articles on J.R.R. Tolkien and His Legendarium. Llyfrawr, 2006, p. 148.
 C.S. Lewis. Studies in Medieval and Renaissance Literature. Cambridge: Cambridge University Press, 1966, p. 28. And for much more on the mythological background of the nicor, including older theoretical underpinnings in Roman and Greek mythology, see Jacob Grimm’s Deutsche Mythologie, especially Vol. II, Ch. XVII.
 E.R. Eddison. The Worm Ouroboros. New York: Albert & Charles Boni, 1926, p. 192.