It's a polite and slightly formal way of disagreeing with someone, often used in a playful or humorous way.
Here's a breakdown:
* "On contrair": This literally translates to "on the contrary".
* "Mon frère": This means "my brother", but it's often used as a term of endearment or to refer to someone in a friendly way, even if they are not actually your brother.
So, you can use this phrase when you want to disagree with someone in a lighthearted way, acknowledging their opinion but then offering a different perspective.