SPQR

Acronym, in use since the days of the ancient Roman Republic, for the Latin phrase: "senatus populusque romanum" ("The Senate and People of Rome").

The acronym has been used continually for approximately 23-24 centuries, and thus qualifies as the oldest acronym in current use.

Though it has been somewhat tainted by association with the fascist government of Benito Mussolini (which harked back to ancient Rome), it remains popular today. The city of Rome, proud of its heritage, still displays it on public works, most notably on manhole covers - despite the fact that the idea of a "Roman Senate" is completely out of date.

The Italians, humorously change the meaning of the ancient Roman acronym SPQR from:

Senatus Populusque Romanus
(The Senate and People of Rome)

to:

Sono Pazzi, Questi Romani.
(The Romans are crazy)

kraken's writeup on SPQR provides an alternate, more virulent revision:

Sono Porci, Questi Romani.
(The Romans are pigs)

By extension (and use of almost any Italian-English dictionary), Italian language nouns and adjectives starting with the letter P may be employed to create one's own versions of this acronym. Here are some examples:

Sono Pigiami, Questi Romani.
(The Romans are pajamas)

Sono Pigne, Questi Romani.
(The Romans are pine cones)

Sono Psichiatri, Questi Romani.
(The Romans are Psychiatrists)

Sono Puntapiedi, Questi Romani.
(The Romans are toe-clips)

Sono Pupazzi, Questi Romani.
(The Romans are snowmen)

Sono Paguri, Questi Romani.
(The Romans are hermit crabs)