Michael Drout was the first to weigh in, suggesting Sméagol derives not from smygel “burrow, cave” but from sméagan “to inquire, investigate, be curious about”, on the grounds that Sméagol was “the most inquisitive member of his community, and he got his name before the murder of Déagol (which means ‘secret’).”
Then, Marcel quoted from the guide to the people, places, and things that Tolkien prepared for translators in the mid-1960’s: “Smials. A word peculiar to Hobbits (not Common Speech), meaning ‘burrow’ […]. It is a form that the Old English word smygel ‘burrow’ might have had, if it had survived. The same element appears in Gollum’s real name, Sméagol.”
To this, I would add that Tolkien had already addressed this point in the appendices to The Lord of the Rings, where he wrote:
This assimilation [i.e., the same as in “the forms and spellings of place-names in Rohan”] also provided a convenient way of representing the peculiar local hobbit-words that were of northern origin. They have been given the forms that lost English words might well have had, if they had come down to our day. Thus mathom is meant to recall ancient English máthm, and so to represent the relationship of the actual Hobbit kast to R. kastu. Similarly smial (or smile) ‘burrow’ is a likely form for a descendant of smygel, and represents well the relationship of Hobbit trân to R. trahan. Sméagol and Déagol are equivalents made up in the same way for the names Trahald ‘burrowing, worming in’, and Nahald ‘secret’ in the Northern tongues. [App. F, II]And there is a relevant passage from Tolkien’s letters also, in a 1967 draft letter to “Mr. Rang”, who had written Tolkien to inquire about his nomenclature. Here, he notes that Old English is the source for:
a few […] survivals in Hobbit-dialect derived from the region (The Vale of Anduin to the immediate north of Lórien) where that dialect of the Northmen developed its particular character. To which may be added Déagol and Sméagol; and the local names Gladden River, and the Gladden Fields, which contains A.S. glædene ‘iris’, in my book supposed to refer to the ‘yellow flag’ growing in streams and marshes. [Letters, #297]Sméagol is not attested in the Old English corpus, though there are many words built from the same roots. In addition to sméagan (as Mike Drout has already noted), there are sméah “creeping in, penetrating”, sméalic “searching, penetrating”, smúgan “to creep, crawl”, sméa(g)ung “search, inquiry, investigation”, etc. But since *sméagol isn’t attested, how do we know whether it would have derived from a noun or a verb. Indeed, how do we know what part of speech *sméagol itself would have been? These questions are at the heart of what Marcel was trying to figure out.
The key lies in its mate, déagol, which is a genuine Old English word. As Mike and others have pointed out, it means “secret, dark, obscure, hidden”, quite suitable for any friend of Sméagol’s. Appropriately enough for an associate of Gollum, déagol also appears in a riddles in the Exeter Book (“Hyrre ic eom heofone, || hateþ mec heahcyning / his deagol þing || dyre bihealdan”). So, if anything can, déagol ought to shed light on *sméagol.
Déagol (also dégol, díegol, dígol, etc.) is an adjective deriving from Primitive Germanic *dauȜilaz. There are cognates in Old High German tougal(i) “hidden”, Old Norse dul “concealment” and dylja “to hide, conceal” (which exhibits palatalization and would probably have been *dylga at some earlier point), and even Modern Swedish dold. The Old English word also survived into Middle English as diȜel. Adjectives ending in –ol normally derive from verbs, and the suffix is indicative of tendency, inclination, ability, etc. Besides déagol (from díegian “to hide” + –ol), there are plenty of other examples: béogol “agreeable”, fretol “greedy”, hetol “hateful”, meagol “earnest, mighty”, numol “capable, nimble”, sprecol “talkative”, sweotol “plain”, þancol “thoughtful”.
This suggests very strongly that *sméagol could well have been a genuine adjective (derived from Primitive Germanic *smauȜilaz), even though it is nowhere recorded. It would apparently have been formed from sméagan “to consider, meditate, examine” + –ol. It probably would not have come from smúgan. That verb could have given rise to an adjective *smugol “creeping, crawling, gliding”, and indeed the Middle English smuhel “lithe, gliding, stealthy, slippery”, a hapax legomenon occurring only in the Ancrene Wisse, is its likely descendant (and cp. Old Irish smugall). This is a word that probably attracted Tolkien’s eye at some point, considering his work on the Ancrene Wisse. But indirectly, the verb sméagan probably did arise metaphorically from smúgan “to creep, crawl”, with its more appropriate sense of Gollumishness. And cp. Old Norse smjúga and of course, Tolkien’s Smaug. Although *sméagol is not recorded, the element sméa– is attested in the adjective sméa-þancol “contemplative, sharp-witted”. And although an adjective *sméagol hasn’t survived in English, at least the noun smygel has. I’ve wondered whether this may be the source of the Germanic/Slavic/Jewish surname Smigel, Smiegel, Schmiegel, etc. Does anyone know?
So, to bottom line this meandering meditation on our slippery friend. The name Sméagol has not been modernized. It retains the form of an unattested but straightforward Old English adjective, precisely analogous to Déagol. And I agree that Mike Drout is probably right that Sméagol derives directly from sméagan (with the addition of a suffix of tendency), and not from smúgan or smygel directly. Moreover, we must conclude that Tolkien was a little bit, well, wrong, when he suggested that Sméagol had to do with burrowing, creeping, worming, etc. Or rather, that sense is there, but it’s buried deeper (appropriately enough), underneath the more immediate sense of inquiring, investigating, being curious about things.