You know a lot about it.
An experienced driver (司机 sī jī) usually has a clear idea of how to get to any place within the city. Therefore, this word is now associated with someone who has a good knowledge of something indecent, which means it often has a derogatory meaning. So, how to say “insider” in Chinese?
Say “Insider” in Chinese: 老司机 (lǎo sī jī) = insider
老 (lǎo) means “old”
司机 (sī jī) means “driver”
We say someone is a 老司机 (lǎo sī jī) to imply that s/he knows something particularly well.