La donna è mobile was a song from the Italian opera Rigoletto by Giuseppe Verdi, from the 19th century.
The Doctor was quite fond of this particular song, and sang it often to himself while doing work in sickbay. On one occasion he was singing it in his office, but his vocal processor cut out because Seven had to temporarily remove his singing abilities. (VOY: "Life Line")
In 2376, while a program the Doctor created to allow him to "day dream" went awry, he imagined himself performing this piece before a large crowd in the messhall, until Tuvok entered Pon farr, and he sang an altered version to distract him. (VOY: "Tinker, Tenor, Doctor, Spy")
Lyrics
La donna è mobile
qual piuma al vento
muta d'accento
e di pensiero
Sempre un'amabile
leggiadro viso
in pianto e in riso
è menzognero
(chorus)
La donna è mobil
qual piuma al vento
muta d'accento
e di pensier
e di pensier
e di pensier
È sempre misero
chi a lei s'affida
chi le confida
mal cauto il core
Pur mai non sentesi
felice appieno
chi su quel seno
non liba amore
Altered Lyrics
Tuvok, I understand.
You are a Vulcan man.
You have just gone without,
For seven years, about.
Paris please find a way,
To load a hypospray.
I will give you the sign,
Just aim for his behind.
Hormones are raging.
Synapses blazing.
It's all so veeery
Illogical.
Illllogical.
Background
The altered version was written by Joe Menosky and small parts by Robert Picardo. Such as Menosky's version had "Paris, if you don't mind, a hypospray you'll find," and Picardo changed it to "Paris please find a way, to load a hypospray."