Misspelling of Christus by some ancient Roman sources. The usage indicates the means of transmission of the text to the modern day. For instance:
"As the Jews were making constant disturbances at the instigation of Chrestus, he (Claudius) expelled them from Rome."
- Gaius Suetonius Tranquillus, The Twelve Caesars
Many scholars feel that Suetonius was referring to the Christians (who were considered a sub-sect of the Jews).
Why the spelling mistake?
The Latin word Christus comes from Χριστος in Greek. The root of the word, χριω, means "to be oily". Although the Greeks rubbed themselves with oil to bathe, the concept of anointment to pass on an office (or divine favor) was alien to them. It's a Jewish custom.
Chrestus, on the other hand, came from Χρηστος, meaning "good", or "worthy".
So when the Romans encountered a cult started by a man known as "the greasy one", or possibly "the guy who just finished his bath", they were sure they were hearing it wrong. These people had to mean "the worthy one", right?
What does it tell us?
Most of our Classical texts come to us through the monasteries. We don't have the originals, nor even contemporary copies. What we have are texts copied out, corrected, amended, and commented on by medieval monks.
Any text in which the term appears (such as The Twelve Caesars) does not come to us through the monasteries. A monastic copyist or scribe would have corrected the text to read Christus.
NB: I find the idea that a separate set of Jews was being agitated by someone named Chrestus, while the followers of Christ were somehow overlooked, an unlikely coincidence. It seems more likely that Suetonius, or one of his sources, got muddled.
However, even if "Chrestus" was not "Christus", the medieval copyists would almost certainly have changed the text, on the supposition that it was. The use proves the non-monastic transmission of the text, whatever its content.