How he could know seems to be a question which can be quite easily answered: although a Roman of high rank usually was supposed to meet with other Romans of high rank, Rome (to begin with) was full of people in the streets who weren't that highly educated. The Fora, the baths, all public places were full of such people: commoners, small farmers and merchants, slaves and freedmen. In the days of Petronius, when the Emperors Claudius and Nero ruled, some freedmen or sons of freedmen could get high ranks at the imperial court. Thus, Petronius, from the time he was Nero's arbiter elegantiae a member of the imperial court himself, could have dealt with freedmen and freedmen's children. Some of those were even in the military, perhaps the ranks of the Cohortes Praetoriae.

How did these Roman freedmen speak? There is some evidence from the 1st century BCE of "common speech" (Sermo Plebeius) through the correspondence of Cicero (106-43 BCE) with some of his friends (especially T.Pomponius Atticus), where Cicero even announces to his friends that he will write to them in the "sermo plebeius" (i.e. the language of the plebs, the common language). So this means, that "common language" was something used by very well educated and literate Romans as well! So what would be the difference between "sermo plebeius" and Vulgar Latin, if any? I think, that "sermo plebeius" could be used by all Romans as "common language", not that grammatically correct, but the distinction between written and spoken language was quite high, higher even than in the most formal languages of today, except, maybe, for Chinese and Japanese. Vulgar Latin can be considered the Latin spoken by people, whose mother tongue was not the Latin language, but any other language or dialect in the Roman Empire, even if it was related to Latin, like any medio-Italic language or dialect.

Petronius may well have known many people, whose mother tongue was not the Latin language and who had to learn this language by themselves or through poor lessons. Everyone had to learn how to speak the language of the Romans and to become a "genuine" Roman citizen by himself.

from left to right: Pompeian wall on the inside and on the outside of a house. Both walls could contain inscriptions or graffiti.

To get an idea what the Vulgar Speech of the Street was like, W. Heraeus (1) made a compilation of inscriptions found on Pompeian walls, which can give us some insight in how the average Pompeian wrote his Latin on the stucco walls of houses and other buildings, for others to read and, often, for himself to joke upon. The inscriptions of Pompeii, a wealthy town of rural and merchant upstarts with lots of money and not that much high standard culture, date from 79 CE. at the latest. Because of the fact that most stucco wall tegmina didn't last longer than ten years, the earliest inscriptions are believed to date from the mid 50's of the 1st century CE. This, in fact, is the period in which Petronius lives and writes. Here are some inscriptions from Pompeii:

I. CIL IV 1824: Quisquis amat veniat. Veneri volo frangere costas

quit ego non possim caput illae frangere fuste?

This is an elegiac distichon (i.e. a combination of two verse lines: a hexameter, followed by a pentameter), but the composer of the poem lost track, because (s)he wrote a hexameter, a pentameter, a hexameter and after that another hexameter! The Latin spelling is almost correct, except for the "quit" in the fourth line, which should be written as "quid". The translation goes like this:

"Whoever is in love, let him come! I want to break Venus' ribs

with my sticks and to crush the goddess's loins.

If she can cut open my tender breast with a knife,

why shouldn't I then be able to break her head with my stick?"

Of course, this poem has a rather double meaning....

II. CIL IV 1880(2): At quem non ceno, barbarus ille mihi est.

"At" should be "ad". Then, the line says: "With whom I don't dine, he is a barbarian to me."

III.CIL IV 1864: Samius Cornelio suspendere.

A very short and sweet message! It says: "Samius to Cornelius, go hang yourself!"

IV.CIL IV 2124: Neroni Caesri Agusto

In some formal inscriptions, people can make mistakes, but on the other hand, shortwriting is a good way to spare the costs of carving letters that can be left out without spoling the message! This reads: "To Nero Caesar Augustus".

V. Not.d. Scavi 1986(3): Sabina, felas, non belle faces.

This must have been written by a common citizen of Pompei, who wanted to scorn someone he knew as a harlot, it says: "Sabina, you s**k (felas instead of fellas), you won't do it nicely (belle is "sermo plebeius" for pulchre).

VI. CIL IV 1173: Quisquis ama valia, peria qui noscit amare,

The first line was obviously written by someone who didn't know any proper Latin; some words are written wrong and not finished, as if (s)he wrote them (perhaps) te way (s)he pronounced them. "ama" should be "amat"; "valia" should be (probably) "valeat"; "peria" should be (probably) "pereat". "Noscit" should (probably) be "nescit", because "noscit" means: "he/she/it learns", which doesn't make sense within the context of the line. The second line could have been written/dictated by someone else with a better knowledge of the Latin language. It says:

"Whoever is in love, may he be well, may he perish who doesn't know to be in love,

may he perish twice as much, whoever forbids it to be in love." (votare= vetare).

Two typical Pompeian indoor graffiti

These are only a few examples of Pompeian inscriptions, which show us some remnants of a Latin spoken by people whose mother tongue was not Latin or who were not that well trained to use the formal Latin language.

Other posts will, hopefully, bring more evidence to light...

Notes:

(1)Heraeus, W., Petronii Cena Trimalchionis nebst Ausgewaehlten Pompejanischen Inschriften, Heidelberg, 1923,

(2)CIL: Corpus Inscriptionum Latinarum, Vol. IV, Berlin 1871.