пленум (plenum) = плен (captivity) + ум (intellect,
wits)
ум + узник (prisoner)* + день (day)** = умник (clever
person; smart Alec) + уздень (Tatar, "chieftain, prince")***
плен + умник = пленник (captive) +
ум
-nik being a personal suffix in Russian, this is
just an alternative (more elegant) way to turn Latin plenum into
Russian plennik.
Btw., there is плен in Гуинплен (Gwynplaine, the
hero of Hugo's L'homme qui rit). Not that I like it, but
пленник + и (and) = Ленин + пик (peak)
*Узница (the word Pushkin used in Andrey Shen'e
to render French la captive) is feminine of узник (prisoner).
Cf. Шильонский узник, Zhukovski's version of Byron's The
Prisoner of Chillon.
**Cf. Ada's Cora Day!
***Обманы хитрых узденей ("the deceptions of
sly chieftains") are mentioned in Pushkin's The Caucasian
Captive. The word уздень is explained in one of the authorial
Notes to the poem.
Alexey Sklyarenko