This phrase is casual and used more in spoken than written English, often in familiar or informal situations.

Colloquial Use

The phrase suggests a deep, thorough understanding, usually from experience or long-term involvement.

Expert Knowledge

It's a figurative expression, meaning there is no actual 'backwards' knowledge involved.

When talking about a person, it implies knowing their habits, personality, and preferences very well.

Often indicates a close emotional bond or relationship where you have a deep understanding of someone.

Like with subjects or places, it's informal and suggests a relaxed comfort with a person's character.