slippery customer—a clever and deceitful person; smb. one should not trust:
- Watch out for that guy with the big padded coat. He may snatch something. He’s a real slippery customer.
smooth customer—an excessively suave or ingratiating person:
- Dobeliou is a smooth customer who does not have to act to seem sincere and trustworthy. He is the perfect choice for a front man to sell the product.
See also: rough customer / tough customer.