General Slyness:
* Rusé: This is a common word for sly, cunning, or shrewd. It implies a cleverness that borders on trickery.
* Finaud: This word emphasizes a playful or mischievous slyness.
* Astucieux: This means "astute" or "cunning" and carries a more positive connotation than "rusé."
Specific Slyness:
* Sournois: This is a more negative term, implying a hidden or deceitful slyness.
* Fichu: This informal word means "sly" or "tricky" and is often used in a playful or ironic context.
Example:
"The rusé fox outsmarted the farmer."
"The finaud child snuck a cookie before dinner."
"The sournois thief stole the wallet without anyone noticing."
The best word to use will depend on the specific context and the nuances you want to convey.