The phrase is often used informally to describe someone's attitude rather than actual knowledge.

Colloquial Usage

The phrase implies overconfidence and can be used negatively to describe arrogance.

Overconfidence

Can be used sarcastically to humorously comment on someone's know-it-all behavior.

Indicates real, extensive knowledge, usually from in-depth study or experience.

It's used to positively recognize someone's expertise or thorough preparation.

Can imply that a person or resource is a reliable and comprehensive source of information.