Here are some synonyms for "knavish":
* Trickster
* Rogue
* Rascal
* Devious
* Cunning
* Sly
* Artful
Example:
"The knavish salesman convinced the elderly woman to buy a useless product."
Origin:
The word "knavish" comes from the Middle English word "knave," which meant "boy" or "servant." Over time, the word developed negative connotations and came to mean "rogue" or "villain."