well-informed person; insider; someone in the know
Refers to a person who is knowledgeable about a particular matter or situation, often implying inside information or familiarity with the background. Can also be used attributively to describe such a person.
See also: 通
He is an insider in this industry.
According to someone in the know, there will be a big announcement next month.
Compound of 事情 (circumstances, situation) and 通 (expert, connoisseur).