According to Bess (which is 'fiend' in Russian), Dan's buxom but otherwise disgusting nurse, whom he preferred to all others and had taken to Ardis because she managed to extract orally a few last drops of 'play-zero' (as the old whore called it) out of his poor body, he had been complaining for some time, even before Ada's sudden departure, that a devil combining the characteristics of a frog and a rodent desired to straddle him and ride him to the torture house of eternity. (Ada, 2.10)

Especially so now - when everything had gone to the hell curs, k chertyam sobach'im, of Jeroen Anthniszoon van Äken and the molti aspetti affascinati of his enigmatica arte, as Dan explained with a last sigh to Dr Nikulin and to nurse Bellabestia ('Bess') to whom he bequeathed a trunkful of museum catalogues and his second-best catheter. (ibid.)

'Play-zero' is a play on plaisir; and Bellabestia means "beautiful beast". In the Foreword to the second edition (1898) of his book Opravdanie dobra (The Justification of Good) V. Solov'yov quotes two German sayings:

Jedes Tierchen hat sein Pläsierchen.* Эта формула представляет истину бесспорную и только требует дополнения другою, столь же бесспорною: Allen Tieren fatal ist zu krepieren.**

Falling in love with Bess, uncle Dan confirms the correctness of Russian saying: sedina v borodu, bes v rebro ("a streak of grey brings an old boy to his spring day").

boroda = dobro + a = dobrota - t (boroda - beard; dobro - good; dobrota - kindness)

Dobro rhymes with rebro (rib). In fact,

dobro + rech' = rebro + doch' (rech' - speech; doch' - daughter)

According to the Soviet (Kaluga-born) poet Kunyaev, dobro dolzhno byt' s kulakami ("good should have fists").

kulak = kukla (kulak - fist; kukla - doll)

On Ada's twelfth birthday Dan gave her a huge beautiful doll:

We cannot reconstitute the exact wording of the message, but we know it said that this thoughtful and very expensive gift was a huge beautiful doll - unfortunately, and strangely, more or less naked; still more strangely, with a braced right leg and a bandaged left arm, and a boxful of plaster jackets and rubber accessories, instead of the usual frocks and frills. (1.13)

It was Bess who helped Dan to choose this thoughtful gift:

And, conversely, Marina refrained from telling Demon about the young hospital nurse Dan had been monkeying with ever since his last illness (it was, by the way, she, busybody Bess, whom Dan had asked on a memorable occasion to help him get 'something nice for a half-Russian child interested in biology'). (1.38)

In his poem "Tsar Nikita and his Forty Daughters" (1822) Pushkin uses dobro in the sense "testicles":

Ukho vsyak derzhal vostro

i khranil svoyo dobro.

Everybody kept a sharp look-out

and treasured his belongings.

Russian for "fist," kulak is Tartar for "ear" (ukho mentioned by Pushkin in the above quoted lines). Uzun-kulak (telegraph, literally "long ear") is mentioned in Ilf & Petrov's "The Little Colden Calf."

Nikitin's poem Kulak (Middleman, 1858) ends in the question:

Когда увидим человека —
Добра божественный сосуд?..

When will we see the man -

A divine vessel of good?

The epigraph to Kulak (in which dobro is also mentioned) is from Romeo and Juliet:***

Все благо и прекрасно на земле,
Когда живёт в своём определенье;
Добро везде, добро найдёшь и в зле.
Когда ж предмет пойдёт по направленно,
Противному его предназначенью,
По сущности добро, он станет — злом.
Так человек: что добродетель в нём,
То может быть пороком.

which seems to correspond to

For naught so vile that on the earth doth live

But to the earth some special good doth give.

Nor aught so good but, strained from that fair use

Revolts from true birth, stumbling on abuse.

Virtue itself turns vice, being misapplied,

Nikitin + ulika + pol = Nikulin + politika (ulika - piece of evidence; pol - floor; sex; politika - politics)

*Every little animal has its little pleasure.
**All animals must die.

***translated - by Pletnyov, I believe - as Romeo i Yulia