Stan Kelly-Bootle:"Again we see a semantic spread that can impact different readers according to their linguistic backgrounds. Both dulce and dolce are familiar to most educated Anglophones...Here the Latin root is dulcis = pleasant/cool, with not a candy-wrapper in sight. English is particularly rich in offering ways of nouning adjectives...VN had a wide choice going from adjective (sweet=dulce) to noun (sweetness-dulc????). Any of –dom, -ness, -ity, -ment, -ence, -ance, -hood, -itude, etc., readily work semantically, however unfamiliar and ugly some of these suffixed nouns may strike the unforgiving prescriptionist...Dulcitude is a pleasant choice, borrowing the natural Latin noun-suffix, -itudo, as found in multitudo."
JM: As SKB notes VN had a wide choice to proceed from adjective to noun, although once again he favored the same "-itudo" pattern, as in Ada's in "mollitude",*  both of which are not a "pleasant choice" in my linguistic background.
Here Nabokov's ploy derives from his attempt to echo Pushkin's strategy. He tries to find a "dead" old word to render "youth"(the end of which Pushkin is deploring) together with its edulcorated evanescence.  I cannot remember now any other example  but I think Nabokov employed this "contrasting" irony at least once in his novels.  
"Dreams, dreams! Where is your dulcitude?/ Where is (its stock rhyme) juventude?"**   
* - Nabokv-L 12/Set/2010 A.Bouazza: "Mollitude is not VN's coinage, but its usage is attested by the OED as early as the 17th century and is defined as "softness, effiminacy". For VN's justifications for using this obsolete word to render the Russian nega, see his commentary to Eugene Onegin. See also "Reply to My Critics" on pp. 244ff of Strong Opinions. The word is, of course, from the Latin mollitudo, which Lewis & Short define as "suppleness, flexibility, softness."
Robert Browning used the adjective mollitious in his long poem Sordello and in The Ring & the Book.
The word was discussed in the past:"
**"The noun mólodost' ('youth,' as a state or a period) has an archaic form, mládost', no longer in use even in poetry[...]   It may be argued that in no age has dulcitude-juventude cropped up commonly in English poetry as sládost' - mládost' did in Pushknin's day and that therefore the analogy is strained....(excerpt from a former posting )
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