(one's) line of country British phrase
A subject one is knowledgeable or likes a lot
Maths is my line of country. I could spend hours unraveling the mathematical puzzle.
Psychology is her line of country. She has treated a number of emotionally disturbed children.
Realizing that cooking is my line of country, I decided to pursue vocational education to be a skilled chef rather than attending university.
I have quitted my current job because it is not my line of country.
Have a strong feeling of affection for someone or something
To have experience and understand of how bad or unplesant something is
If you are in/get into somebody's good/bad books, you have/not have their favour.
To give someone or something better treatment than others.
Know a lot about something
The origin of this phrase is not clear.