IPA: /nɒnˈdʒɔɪnər/
KK: /nɑnˈdʒɔɪnər/
A person who chooses not to join a group, organization, or movement.
As a nonjoiner, she preferred to work independently rather than be part of a team.
Nonjoiner → It is formed from "non-" (meaning not) and "join" (from Old French *joindre*, meaning to connect or link) and "-er" (meaning a person who). A nonjoiner is a person who does not connect or link with others.
Think of someone who is 'not' ('non-') connecting or 'joining' with others — that's what a nonjoiner means.