the.com/insider

means Someone who belongs to a group closely enough to have access to its private information, decisions, or workings.

from A straightforward compound of 'inside' plus the agent suffix '-er,' marking one who is within something rather than outside it. The spatial metaphor — inside the walls, inside the circle, inside the room where it happens — did the real work; by the 19th century 'insider' had settled into its modern sense of someone with privileged access or knowledge, often contrasted pointedly against the 'outsider' left at the door.