In any case, Tristan sings this peculiar word in the course of his dialogue with Kurvenal in Scene 1 of Act III. Having been mortally wounded at the end of the previous act, Tristan is awoken by the shepherd's “alte Weise” in Kareol and is greeted by Kurvenal's optimistic forecast of their future there. Yet Kurvenal's words fall on deaf ears, as Tristan can only think of the hereafter. The latter launches into a quasi-soliloquy that describes what sounds to modern ears like a near-death experience: “Ich war, wo ich von je gewesen, wohin auf je ich geh': im weiten Reich der Weltennacht” (“I was where I had been before I was and where I am destined to go, in the wide realm of the night of the world”). Paradoxically, all one knows in this Weltennacht is “göttlich ew'ges Urvergessen” (“primal forgetfulness, divine and eternal”), a kind of not-knowing.
Wagner's fascination with this sort of wisdom in un-wisdom stemmed of course from his exposure to Schopenhauer, who believed that the essence of reality (the Will) is more accessible via the routes of un-reason (like music) than those of reason. In death, Schopenhauer thought, the Will's veil is pulled back and man is liberated from his corporal limitations. Thus we can understand the ambivalence inherent in Tristan's “Todeswonne-Grauen” (“terror of death's bliss”), an ambivalence rooted in the conflict between the human instinct for survival and the spiritual desire for meta-human enlightenment. More than anywhere else in the libretto, it is in this mystical, near-death reminiscence that the opera's philosophical foundation comes into focus.
As one might expect, the music that sets and accompanies these lines serves to highlight their dramatic importance. Not long into his solo, Tristan reaches the key of F minor, which has been the anchor point of the act so far (Kurvenal's optimism has been marked by major key excursions, particularly to F major). Yet by “Ich war, wo ich von je gewesen…” (see above), which is sung to a melody associated with the Weltennacht, Tristan has moved to A♭ minor. Over a dominant pedal he sings “göttlich ew'ges,” paraphrasing the Desire motive à la the Act III prelude. Yet Tristan's climactic “Urvergessen!” (at 228/4/4 in the Schirmer vocal score) provides a jolt, both textually and musically:
|Richard Wagner, Tristan und Isolde, Act III, Scene 1, 228/4/1–5.|
Chafe hears in these measures “a paradoxical statement matched by perhaps the most mysterious enharmonic relationship in the opera” (318). The latter refers to his analysis of the unexpected D major as an enharmonic E♭♭ major (245). I'm more inclined to take the D major at face value and instead analyze 228/4/3's D♭ as an enharmonic C♯ (i.e., as some bastardized augmented-sixth chord leading to the dominant of G). In some sense this interpretation can be reconciled with the Desire music that follows, which half-cadences in A, C, and E via the Tristan chord, which is itself a kind of bastardized French augmented-sixth. Taken together, these four tonics—A, C, E, and G—constitute what Robert Bailey refers to as the A/C “double-tonic complex,” or the grouping of two tonalities a minor third apart into a kind of meta-tonality, such that “either [tonic] can serve as the local representative of the tonic complex” (121–22). Though he identifies A minor-seventh as the “harmonic embodiment" of the A/C complex (given its superimposition of A minor and C major), in fact the keys of A and C are of indeterminate quality in Bailey's scheme (“we are now dealing with the ‘chromatic’ mode of A and the ‘chromatic’ mode of C”). He identifies the A/C complex as an important factor in the large-scale organization of the Tristan “Prelude" and Act I, but is it also a factor here? I dunno; but however you want to analyze it, what one expects on the downbeat of 228/4/4 is E♭ major (or E♭ dominant-seventh), not D major.
Yet, despite the resonance of this foreign harmony with the bizarre notion of an Urvergessen, in some sense it is the melodic character of 228/4/4–5 that most reflects the text. There is really nothing more primordial sounding in a tonal context than the “arpeggiation" of an open fifth. Indeed, with its dotted rhythms this passage calls to mind the creatio ex nihilo of Beethoven's Ninth. The character of this “Urvergessen!” motive is mildly foreshadowed in the earlier line “im weiten Reich der Weltennacht,” (from 228/3/1) particularly in its final A♭4–D♭5–D♭4, but the former is much more austere, owing to its greater directness (i.e., fourth–fifth rather than fourth–octave). Though bare octaves are in a sense more “primal" than bare fifths, they don't really register as such because of the principle of octave equivalence, according to which the octave is not an interval at all (at least in pitch-class space). By contrast, the bare fifth—in a tonal context—implies a reduction of the triad to its skeleton, to its primary orienting interval. This effect is only heightened by Tristan's mature Wagnerian harmony, in terms of which an open fifth implies a stripping away of not only the third, but possibly also the seventh and the ninth.
There is certainly more to be said about this wonderful moment in what is probably my favorite opera, but for now the above will have to suffice. In summary, the shock that comes with Tristan's “Urvergessen!” is threefold: the D-major triad marks a disruption of the harmonic flow; Tristan's traversal of a fifth-split octave marks a stark departure from the default melodic idiom; and the word Urvergessen itself—in its novelty and paradoxical usage—marks a break from rational, human thought. Through this truly musico-logical rupture, Wagner allows us a glimpse of the Weltennacht, where the shackles of body and mind are dissolved in the Will.
 This dialogue parallels that of Brangäne and Isolde in Act I, Scene 3.
 My German is very bad. This translation is taken from RWagner.net, though I can't trace its provenance any further than that.
 The Desire motive is the one that opens the entire opera (the rising chromatic tetrachord G♯–B, with its characteristic rhythm). The Desire music, by contrast, is the first seventeen measures of the opera.
 … and by extension I would analyze the C♭ as a B♮ and possibly the F as an E♯ (though maybe that's a little pedantic).
 “An Analytical Study of the Sketches and Drafts," from Bailey's 1985 edition of the Prelude and Transfiguration from Tristan und Isolde.