A silver/smooth tongue phrase
If someone "has a silver/smooth tongue", he/she is able to speak in a clever and well-spoken manner that pleases people and makes them do what he/she wants.
It is Tom's silver tongue that makes his partner sign the contract.
Melinda has a smooth tongue. She can convince her boss of changing his mind.
A silver tongue is what a salesman needs to persuade the customers.
The first use of the phrase appeared in the 1590s. Today, most know "silver" to mean a metallic color. However, there is actually another definition that is not widely known defining "silver" as a word to describe the sound of something pleasing
People tend to like forbidden or illegal things just because they are forbidden or illegal.
He must have believed that stolen fruit is the sweetest, and he, therefore, cheated on his partner.