IPA: /ˈsaɪdˌlaɪnər/
KK: /ˈsaɪdˌlaɪnər/
A person who does not take part in an activity or event, often observing from the side instead of participating.
During the game, he was just a sideliner, watching his friends play without joining in.
Sideliner → It is formed from "side" (from Old English *side*, meaning the lateral part) and "liner" (from the verb "line" meaning to mark or delineate). The word "sideliner" refers to someone who is on the side, often not actively participating in the main action, but observing or waiting.
Think of someone who is on the 'side' and 'lining' up to watch or wait — that's why a sideliner is someone who observes from the sidelines.