Is there any chance the words "leithian" (release from bondage), "leithia" (release), "leith" (set free), etc. might find a sort of auditory cognate in, or rest upon, or take some sense from the Greek "lēthē" (a forgetting, forgetfulness; in the post-Homeric period, a place of oblivion in the lower world). How does one pronounce "leithian"? Does the "th" sound like the "th" in "either" or "ether" (note the small difference)? Might Tolkien's "leithian" be related in any way (even a distant way) to the little-known and rarely used English word "lethean" (causing oblivion or forgetfulness of the past)? Mightn't there be a verbal echo at least? A forgetting is a release; forgetting one's mortal life is a kind of release from the bondage of mortality. Am I on the right track here? Am I on one of perhaps many tracks? How far afield am I in positing Greek (or a tiny part of it) as a foundational language for Tolkien's extensive and expansive invented vocabulary? I don't mean to suggest that we can (or should) trace Tolkien's words to actual Greek roots, only that we might proceed with a remnant Greek vocabulary echoing in the mind. Mightn't Tolkien have had remnants of his own Greek echoing through his mind? (Of course we cannot know for certain.) I'd appreciate any insights from those of you who are far more familiar with Tolkien than I am. I'm a beginner, as my questions might suggest.
Tolkien's word "leithian" and associated forms (e.g., "leithia," "leith")
As you say, both Greek and older forms of English were familiar to Tolkien (among many other natural languages), so it's certainly possible that sound shapes from both came to bear upon his own linguistic creations. There are dozens of other examples of Eldarin words with 'real-world' equivalents; the most immediate to my mind is Finnish antaa 'to give' ~ Quenya anta- of the same meaning. I don't think we can read much further into it than that, though, but that's just one lambengolmo's opinion.
I would agree with Röandil but I should also share this from Tolkien's earlier works: Sindarin's draft form "Goldogrin" had leithia- as...
G. laith(r)a-, v. “to let slip, lose, mislay, forget; (intr.) to be lost”
And of course, we cannot trace Tolkienian words back to real languages, but there are certainly a few inspirations, and this seems to be one of them. Another one that I recently noticed was Proto-Germanic *auzi "dawn" (Old Norse ?aur) which we see in Q. aurë (former root form "auri-") & S. aur "day".