"老司机" (lao si ji) refers to the people who are steady and have rich experience in certain fields. It is also often used to refer to the people whose stock resources are rich. The people who are familiar with certain aspects and have older qualifications, broad knowledge and enough experience can be called "老司机" (lao si ji). It is widely used on post bars and forums.
The features that this kind of "老司机" (lao si ji) may have are:
Large stock of resources;
Willing to share their resources with others;
They read a lot of resources, being able to recognize many books or films and "departing".
The categories of their resource collections are special, or heavier taste.